Physical 
ScLLib. 

QD 
S43 

M6 


B   M   ISM 


THE 


.OTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS 


REPORT  ON 


INVESTIGATIONS  MADE  IN  THE  CHEMICAL  LABORATORY 

OF  THE  JOHNS  HOPKINS  UNIVERSITY 

DURING  THE  YEARS  1899-1913 


BY  H.  N.  MORSE 

Professor  of  Inorganic  and  Analytical  Chemistry  in  the  Johns  Hopkins  University 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 

1914 


UNIVERSITY  FARM 


QDS4-3 

tu 


THE 
OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS 


REPORT  ON 


INVESTIGATIONS  MADE  IN  THE  CHEMICAL  LABORATORY 

OF  THE  JOHNS  HOPKINS  UNIVERSITY 

DURING  THE  YEARS  1899-1913 


BY  H.  N.  MORSE 

Professor  of  Inorganic  and  Analytical  Chemistry  in  the  Johns  Hopkins  University 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 

1914 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  198 


PRESS  OF  GIBSON  BROTHERS,  INC. 
WASHINGTON,  D.  C. 


CONTENTS. 


Chapter  I.  Page. 

The  cells  and  the  manometer  attachments 3 

Treatment  of  the  clays 6 

First  process 7 

Second  process 7 

The  formation  of  the  cylinders 8 

The  cutting  of  the  cells 11 

The  burning  and  glazing  of  the  cells 12 

The  manometer  attachments  of  the  cells 17 

Chapter  II. 

The  manometers 27 

Purification  of  the  mercury 28 

Calibration  of  the  manometers 28 

First  method 29 

Second  method 30 

The  meniscus 33 

The  uncalibrated  portions  of  the  manometers 37 

Capillary  depression 39 

The  filling  of  the  manometer 45 

Determination  of  the  volume  of  the  nitrogen 48 

Chapter  III. 

The  regulation  of  temperature 51 

Thermometer  effects 51 

The  scheme  for  electrical  regulation 57 

The  battery 59 

The  thermostat 59 

The  master  relay 61 

The  minor  relay 61 

The  bath  for  0° 62 

Baths  for  maintenance  of  temperature  above  zero 63 

Type  1 65 

Type  II 66 

Type  III 68 

Type  IV 71 

Chapter  IV. 

The  membranes 77 

The  deposition  of  the  membrane 82 

Observations  on  the  membrane 85 

Temperature  of  deposition 85 

Treatment  of  the  cell  while  in  use 86 

The  soaking  of  the  cell 87 

Activity  of  the  membrane 88 

Deterioration  of  the  membrane 91 

Effect  of  the  electrolytes 92 

Semipermeability  of  membranes 92 

Removal  of  the  membrane 93 

Infection  of  the  membrane 94 

Chapter  V. 

The  weight-normal  system  for  solutions 97 

Chapter  VI. 

Cane  sugar Ill 

Preliminary  determinations  of  osmotic  pressure Ill 

Series  1 112 

Series  II 118 

Series  III 128 

Series  IV 132 

Series  V 135 

Series  VI 139 

Series  VII 140 

Series  VIII .  .                                                                                                142 


IV  CONTENTS. 

Chapter  VII.  Page. 

Glucose 151 

Preliminary  determinations  of  osmotic  pressure 151 

Series  1 151 

Series  II 154 

Series  III 156 

Chapter  VIII. 

Cane  sugar 159 

Final  determinations  of  osmotic  pressure 159 

Chapter  IX. 

Glucose 188 

Final  determinations  of  osmotic  pressure 188 

Chapter  X. 

Mannite 197 

Determinations  of  osmotic  pressure 197 

Chapter  XI. 

Electrolytes 209 

Experiment  1 211 

Experiment  2 212 

Determinations  of  the  osmotic  pressure  of  lithium  chloride 214 

Chapter  XII. 

Conclusion 221,  222 


LIST  OF  ILLUSTRATIONS. 


PLATES-  Facing 

Page. 
Plate  1.  a,  c,  and  e,  thin  sections  taken  from  potters'  cells,    b,  d,  and/,  thin  sections  taken 

from  cells  made  in  the  laboratory 16 

Plate  2.  View  of  "manometer  house,"  cathetometer,  arrangement  for  pressing  clays,  and 

one  style  of  rectangular  bath 42 

Plate  3.  View  of  circular  and  rectangular  laboratory  baths  in  use 68 

Plate  4.  Rectangular  bath,  end  view 70 

Plate  5.  Rectangular  bath,  side  view 72 

TEXT  FIGURES. 

Page. 

1.  Steel  press  for  clays.     Ball-bearing  disk 8 

2.  Apparatus  for  pressing  clays 10 

3.  Clay  cylinder  after  pressing.     Clay  cell  after  shaping  the  cylinder  on  the  lathe 11 

4.  Different  views  of  special  tool  for  cutting  cell  from  cylinder 12 

5.  Electric  kiln  for  baking  cells.     Inner  and  outer  coverings  for  electric  kiln 14 

6.  Electric  kiln  arranged  as  crucible  furnace 14 

7.  First  form  of  complete  cell 19 

8.  "Fang"  for  introduction  and  removal  of  rubber  stoppers 18 

9.  Second  form  of  complete  cell 19 

10.  Third  form  of  complete  cell 22 

11.  Fourth  form  of  complete  cell 22 

12.  Fifth  form  of  complete  cell;  for  use  with  substances  which  attack  metals 23 

13.  Solid  glass  stopper  for  use  with  substances  which  attack  metals 25 

14.  15.  Glass  manometer  attachments  for  cells  with  straight  necks 25 

16.  First  arrangement  for  calibrating  manometers 30 

17.  Second  arrangement  for  calibrating  manometers 31 

18.  Simplest  form  of  manometer 32 

19.  Manometer  for  high  pressure 32 

20.  Manometer  with  glass  cone  for  cells  with  taper  necks 36 

21.  Manometer  with  glass  connection  for  cells  with  straight  necks 38 

22.  "Steel  block"  for  determination  of  gas  volumes  in  manometers,  for  comparison  of 

instruments,  and  for  determination  of  capillary  depression 40 

23.  Electric  hammer  for  tapping  manometers 41 

24.  "Manometer  house"  for  calibration  and  comparison  of  instruments,  etc 43 

25.  Improvement  in  cathetometers  for  the  fine  adjustment  of  the  telescope,  which  also 

serves  as  a  substitute  for  the  micrometer  eye-piece 44 

26.  Arrangement  for  filling  manometers  with  nitrogen 46 

27.  "Brass  block" 49 

28.  Apparatus  used  in  emptying,  filling,  and  cleansing  the  uncalibrated  portion  of  the 

manometers 50 

29.  General  scheme  for  the  electric  regulation  of  bath  temperature 58 

30.  The  thermostat 60 

31.  Interior  ice-bath  for  measurement  of  osmotic  pressure  at  0° 62 

32.  60-liter  galvanized-iron  bath  for  intermittent  use 63 

33.  Coil  of  block-tin  pipe  for  cooling  or  heating  water  before  it  enters  the  circulating  system 

within  the  bath 66 

34.  Rectangular  bath  for  general  laboratory  use 67 

35.  36.  Lower  and  upper  halves  of  rectangular  bath  for  measuring  osmotic  pressure 69 

37.  Hot-water  circulating  system  with  end  of  bath  removed 70 

38,  39.  Brass  and  copper  bath  for  high  temperature  work 72 

40.  Brass-copper  bath  for  high  temperatures 73 

41.  Exterior  view  of  bath  for  high  temperatures 74 

42.  View  between  interior  and  exterior  baths,  i.  e.,  of  space  filled  with  water 74 

43.  Automatic  arrangements  for  maintaining  temperature  of  upper  door  when  open 75 

44.  Larger  (elliptical)  bath  for  high  temperatures 75 

45.  Exterior  view  of  larger  bath  for  high  temperatures 76 

46.  First  bath  employed  for  measurement  of  osmotic  pressure 114 

47.  First  bath  in  which  water  and  air  were  circulated 119 

48.  Pumping  arrangements  on  larger  scale  than  in  figure  47 120 

49.  Interior  view  of  water  compartment  with  covers  partly  removed 121 


THE  OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS 


BY  H.  N.  MORSE 


CHAPTER  I. 
THE  CELLS  AND  THE  MANOMETER  ATTACHMENTS. 

Having  found  in  the  electrolytic  method*  an  excellent  means  of 
depositing  a  considerable  number  of  osmotically  active  membranes,  it 
was  imagined  that  the  principal  obstacle  in  the  way  of  the  measure- 
ment of  osmotic  pressure  had  been  removed  and  that  certain  obvious 
mechanical  difficulties  connected  with  the  preparation  of  a  suitable 
porous  vessel  and  the  assembling  of  the  various  essential  parts  of  the 
cell  could  be  readily  overcome.  It  was  soon  discovered,  however,  that 
the  problem  of  preparing  a  satisfactory  background  for  the  membrane, 
i.  e.,  the  porous  wall,  was  vastly  more  difficult  than  had  been  antici- 
pated.! It  was  necessary,  in  fact,  to  spend  a  large  fraction  of  the  first 
ten  years  of  the  investigation  in  experimental  work  in  the  manufacture 
of  cells.  The  first  four  years  (1901-1905)  were  devoted  almost  exclu- 
sively to  the  solution  of  that  problem.  At  the  close  of  the  latter  period 
(1905),  only  two  porous  vessels  of  faultless  wall-structure  had  been  pro- 
duced. These  were  the  cells  which  were  designated  in  the  published 
records  of  the  work  by  the  letters  "A"  and  "B." 

The  first  experiments  upon  the  activity  of  membranes  deposited  by 
the  electrolytic  method  were  made  in  such  porous  vessels  as  could  be 
found  about  the  laboratory,  battery  cups,  etc.  The  earliest  attempts 
at  quantitative  measurement  were  carried  out  with  a  portion  of  a  lot 
of  100  small  porous  cups  which  were  manufactured,  in  accordance 
with  furnished  specifications,  at  a  pottery  in  a  neighboring  city.J  In 
about  one-fourth  of  these,  considerable  pressures  were  developed,  but 
in  no  case  the  maximum  pressure.  All  of  them  leaked,  and  most  of 
them  burst  under  pressures  of  less  than  20  atmospheres.  Only  one 
of  them  survived  a  pressure  of  30  atmospheres,  and  that  for  a  short 
time  only.  It  was  not  then  doubted  that  the  defects  which  had 
appeared  in  the  first  lot  of  cells  from  the  pottery  could  be  remedied 
by  the  potters  themselves,  provided  the  exact  causes  of  the  failure  of 
their  products  could  be  correctly  ascertained  and  explained  to  them. 
Accordingly,  with  that  purpose  in  view,  many  thin  sections  were  made 
of  the  cells  with  which  quantitative  measurements  had  been  attempted, 
and  these  were  examined  microscopically  and  photographed.  It  soon 
appeared  that  most,  if  not  all,  of  the  conditions  which  determine  the 

*Amer.  Chem.  Journal,  xxvt,  80  (1901);  xxix,  173  (1903). 
t/Wd.,  xxxii,  93  (1904);  xxxiv,  1  (1905). 
\Ibid.,  xxviii,  1  (1902). 


4         OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

good  and  bad  behavior  of  the  porous  wall  of  the  cell  are  susceptible  of 
clear  definition  and  of  adequate  explanation.  All  the  essential  facts 
which  had  been  discovered  in  the  laboratory  were  given  to  the  potters, 
and  they  expressed  their  confidence  in  their  ability  to  remedy  the 
defects  of  the  earlier  consignment.  In  this  expectation  they  were 
greatly  mistaken;  for,  among  the  nearly  500  cells  which  were  subse- 
quently made  for  us  at  various  potteries,  not  one  was  found  suitable  for 
the  measurement  of  osmotic  pressure.  In  fact,  in  attempting  to  remedy 
certain  defects,  they  generally  aggravated  others  to  such  an  extent  as  to 
render  the  later  cells  on  the  whole  distinctly  inferior  to  those  of  the  first 
lot.  Finally  we  ventured  to  offer  certain  suggestions  involving  methods 
of  manufacture  not  in  use  among  potters,  but  these  were  rejected  on 
the  ground  that,  to  those  familiar  with  the  conduct  of  clays,  they  were 
obviously  futile.  As  the  potters  declined  to  cooperate  along  lines  of 
manufacture  not  approved  by  them,  the  problem  of  cell-making  was 
taken  out  of  their  hands  into  the  laboratory  for  solution. 

The  cells  produced  at  the  potteries  were  defective  in  various  ways, 
but  principally  in  the  particulars  enumerated  below: 

1.  All  were  lacking  in  the  strength  necessary  to  withstand  any  con- 
siderable outward  pressure.     As  mentioned  above,  only  one  of  the  few 
which  proved  at  all  serviceable  survived  a  pressure  of  30  atmospheres, 
while  most  of  them  cracked  under  pressures  below  20  atmospheres. 

2.  All  of  them  contained  numerous  "air  blisters,"  which  communi- 
cated with  each  other  and  with  the  interior  surfaces  of  the  porous  wall 
in  such  ways  as  to  give  rise  to  the  formation  of  a  number  of  subsidiary 
interior  membranes.     Not  unfrequently,  when  a  cell  was  broken  for 
examination,  as  many  as  four  or  five  of  these  minor  membranes,  often 
nearly  concentric  over  a  considerable  area,  were  found  in  several  local- 
ities; and  it  frequently  happened  also  that  the  last  of  them  was  near,  or 
even  at,  the  exterior  surface  of  the  cell. 

3.  The  potters'  cells  also  lacked  uniformity  in  respect  to  porosity. 
The  same  cell  would  often  exhibit  the  greatest  diversity  in  this  par- 
ticular.    In  some  parts,  the  structure  would  be  as  close  as  in  porcelain, 
while  in  others  it  might  be  so  open  that  the  membrane  would  form 
nearly  midway  between  the  interior  and  exterior  surfaces  of  the  cell 
wall. 

A  miscroscopic  examination  of  thin  sections  of  the  cells  in  which 
membranes  had  been  deposited  revealed  the  fact  that,  excluding  the 
peculiar  and  often  fantastic  effects  of  "air  blisters"  the  distance  of  the 
membrane  from  the  interior  surface  of  the  cell  wall  is  determined  solely 
by  the  porosity  of  the  latter.  The  more  open  the  texture  is,  i.  e.,  the 
larger  the  pores  are,  the  more  deeply  within  the  wall  will  the  deposition 
occur;  while  with  a  certain  degree  of  closeness  in  this  respect,  the  depo- 
sition is  just  within  the  interior  entrances  of  the  pores,  in  effect,  upon  the 
inner  surface  of  the  cell,  where  it  should  be.  Obviousjy  the  copper 


CELLS   AND   MANOMETER   ATTACHMENTS.  5 

ferrocyanide  membrane  will  always  be  located  somewhat  nearer  the 
inner  than  the  outer  surface  of  the  wall,  however  large  the  pores  may  be. 
It  was  evident  that  the  hope  of  success  in  cell-making  depended  on 
the  following  conditions: 

1.  Great  and  uniform  strength  of  wall. 

2.  The  elimination  of  air-blisters. 

3.  An  excessively  fine  and  perfectly  uniform  texture  of  wall,  a  texture 
so  fine,  in  fact,  as  to  insure  the  meeting  of  the  slower  anion  and  the 
more  mobile  cation  just  within  the  interior  mouths  of  the  pores.     In 
other  words,  the  pores  must  be  so  small  that  the  cation  is  able  to  pass 
through  them,  from  the  exterior  to  the  interior  of  the  wall,  during  the 
time  consumed  by  the  anion  in  just  entering  them  from  the  interior. 

The  necessity  of  securing  great  strength  of  wall  is  obvious  enough,  as 
is  also  that  of  eliminating  "air  blisters,"  and  the  need  of  depositing  the 
membrane  at  the  interior  surface  of  the  wall  will  likewise  become  appar- 
ent if  one  considers  the  inequalities  in  the  concentration  of  the  solution 
which  must  result  from  its  location  elsewhere,  i.  e.,  within  the  wall.  In 
the  latter  case,  owing  to  the  slowness  of  diffusion  within  the  wall,  the 
liquid  in  the  neighborhood  of  the  membrane  will  be  permanently  less 
concentrated  than  the  main  body  of  the  solution.  Moreover,  since  the 
wall  is  always  necessarily  filled  with  some  liquid,  it  would  be  impossible 
to  know  exactly  the  final  concentration  of  any  solution  which  is  intro- 
duced into  the  cell.  On  the  other  hand,  if  the  discharge  of  the  water 
entering  the  cell  through  the  membranes  is  from  a  free  surface,  i.  e., 
directly  into  the  unencumbered  solution,  the  conditions  will  be  favor- 
able to  its  rapid  distribution,  and,  therefore,  to  the  maintenance  of 
uniform  concentration. 

It  was  attempted  to  secure  strength  of  wall  by  introducing  into  the 
clays  the  maximum  allowable  portion  of  cementing  material  (feldspar) — 
that  proportion,  in  fact,  which  is  just  insufficient  to  convert  the  baking 
cell  into  porcelain.  It  was  hoped  also,  by  thorough  mixing  of  the  con- 
stituents, to  secure  a  more  uniform  texture  of  cell  wall  than  had  been 
found  in  the  products  of  the  potters. 

Washed  clays  from  several  sources  were  mixed  with  varying  quanti- 
ties of  ground  feldspar,  and  the  mixtures  were  burned  at  different  tem- 
peratures, either  in  a  Seger  experimental  kiln  with  use  of  Seger  cones, 
or  in  a  calibrated  electric  furnace  which  was  devised  for  the  purpose. 
The  products  were  altogether  disappointing.  They  were,  in  reality, 
quite  as  uneven  in  respect  to  uniformity  of  strength  and  texture  as  the 
cells  of  the  potters.  The  failure  was  evidently  due  to  imperfect  mixing, 
and  it  was  hoped  that  better  results  might  be  obtained  with  finer  mate- 
rials. Accordingly,  both  the  clays  and  the  feldspar  were  elutriated,  and 
the  wet  mixtures  of  the  finer  materials  thus  obtained  were  passed 
repeatedly  through  silk  bolting-cloth  having  16,000  holes  to  the  square 
inch.  The  bolting  process  was  followed  by  a  long-continued  churning 


6         OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

of  the  mixtures  with  water,  and,  finally,  by  a  most  thorough  kneading 
of  the  "putty."  The  results  were  still  unsatisfactory  in  that  the  poros- 
ity of  the  baked  samples  lacked  the  high  degree  of  uniformity  which  is 
indispensable  in  the  measurement  of  osmotic  pressure.  It  was  evident, 
moreover,  on  comparing  our  products  with  those  of  the  potters,  that 
we  had  been  trying  exactly  what  they  had  attempted,  except  that,  in 
every  case  but  one,  they  had  omitted  the  elutriation  and  bolting  pro- 
cesses. It  was  concluded  that  the  necessary  binding  material  can  not  be 
successfully  incorporated  with  the  clays  in  the  form  of  ground  feldspar. 

The  final  solution  of  the  problem  was  easy  and  satisfactory.  It 
occurred  to  us  that  perhaps  sufficiently  intimate  admixtures  could  be 
obtained  by  bringing  together  two  different  clays,  one  of  which  is  defi- 
cient in  binding  material,  while  the  other  is  over  rich  in  that  constituent. 
This  was  the  plan  which  was  finally  adopted,  and  with  proper  selection 
and  manipulation  of  the  materials,  it  has  never  failed  to  give  products 
which  are  all  that  could  be  desired  in  respect  to  strength  and  uniformity 
of  texture. 

The  pores  were,  however,  still  much  too  large,  notwithstanding  the 
fineness  of  the  materials,  and  the  air  blisters  were  not  eradicated  by  the 
usual  method  of  forming  such  vessels.  It  was  attempted  to  diminish 
the  size  of  the  pores  by  repeatedly  burning  the  cells  at  high  temper- 
atures, and  in  this  way  considerable  but  not  sufficient  improvement 
was  effected.  Two  plans  had  been  proposed  to  the  potters  for  securing 
the  required  density  of  texture  and  for  the  simultaneous  elimination  of 
the  air  blisters.  The  first  of  these  was  to  form  the  cell  itself  under  high 
pressure,  while  the  second  was  to  form  the  wet  clay  into  a  cylinder  under 
great  pressure,  and  from  this  to  turn  out  the  cell  upon  the  lathe.  Both 
plans  were  declared  to  be  impracticable  by  the  potters.  After  many 
months  of  futile  effort,  we  were  forced  to  agree  with  them  as  to  the  first 
project,  but  the  alternative  plan — that  of  cutting  the  cell  from  a  cylinder 
which  had  been  formed  under  high  pressure — was  finally  developed  to  a 
successful  issue. 

TREATMENT  OF  THE  CLAYS. 

A  considerable  number  of  clays,  both  American  and  foreign,  were 
investigated  with  reference  to  their  suitability  for  the  manufacture  of 
cells,  and  two  were  finally  selected  as  being  superior  to  any  of  the  others 
for  the  purpose.  These  were  a  fire  clay  from  Dorsey,*  Maryland,  and  a 
so-called  ball  clay  from  Edgar,  Florida. 

The  Florida  clay  had  been  washed  before  it  came  into  our  hands, 
while  that  from  Maryland  was  in  its  original  untreated  condition.  Two 
processes  have  been  employed  for  the  separation  of  the  finer  portions  of 
the  clays.  Both  give  satisfactory  products,  but  the  earlier  process  has 
been  abandoned  because  the  later  one  is  more  economical  of  material. 

*Erroneously  stated  to  have  been  from  Mount  Savage,  Maryland. 


CELLS   AND    MANOMETER    ATTACHMENTS.  7 

FIRST  PROCESS. 

The  dry  and  pulverized  clays  are  sifted  for  the  purpose  of  removing 
the  coarsest  parts.  Three  empty  alcohol  barrels,  each  with  a  spigot 
in  the  bung  hole,  are  placed  one  above  another,  each  of  the  upper  two 
being  set  a  little  back  of  the  one  below  it.  The  uppermost  barrel  is 
nearly  filled  with  water,  and  into  this  is  stirred  about  3  kilograms  of  the 
sifted  clay.  After  standing  quietly  for  3  minutes,  the  spigot  is  opened 
and  the  contents  of  the  upper  half  of  the  barrel  are  allowed  to  flow  into 
the  barrel  below.  The  residue  is  removed  and  the  barrel  is  recharged 
and  again  partially  emptied,  precisely  as  in  the  first  instance.  When 
the  intermediate  barrel  is  nearly  full,  its  contents  are  likewise  stirred 
and  then  allowed  to  settle  for  3  minutes,  after  which  the  spigot  is  opened 
to  allow  the  contents  of  the  upper  half  to  flow  into  the  lowest  recep- 
tacle. The  material  which  collects  in  the  lowest  barrel  is  bolted  (wet) 
successively  through  Nos.  10,  14,  and  16  silk  bolting-cloth,  having 
respectively  11,236,  19,600,  and  24,336  holes  to  the  square  inch.  The 
proportion  of  the  clay  which  is  thus  acquired  is  not  very  large.  In  one 
instance  where  the  original  and  final  weights  were  recorded,  500  pounds 
of  the  fire  clay  yielded  180  pounds  of  the  bolted  material.  In  another 
case,  200  pounds  of  the  Edgar  clay  gave  75  pounds  of  the  final  product. 

SECOND  PROCESS. 

A  wooden  trough,  6  meters  in  length,  with  flat  bottom  and  high  sides, 
is  divided  into  several  compartments  by  means  of  transverse  dams. 
The  trough  is  given  an  inclined  position,  and  in  the  highest  compart- 
ment the  sifted  clay  is  stirred  up  with  water.  The  water  with  its  sus- 
pended matter  is  pushed  from  time  to  time  over  the  dam  into  the  next 
compartment.  By  repeating  the  operation  in  the  successive  divisions, 
the  finer  constituents  of  the  clay  can  be  quickly  and  quite  completely 
separated  from  the  coarser.  The  material  which  collects  in  the  last 
compartment,  or  is  allowed  to  overflow  from  that  into  other  receptacles, 
is  bolted  in  the  manner  described  above. 

The  bolted  clay  is  allowed  to  subside  and  the  nearly  clear  water  above 
it  is  drawn  off  by  means  of  a  siphon,  but  there  still  remains  a  large  quan- 
tity of  water  in  the  clay  which  must  be  removed  by  evaporation,  or 
filtration,  or  by  other  means.  Its  removal  by  either  of  the  methods 
mentioned,  however,  is  exceedingly  slow  and  hi  many  ways  disagreeable. 
A  much  better  and  more  rapid  method  is  that  which  was  suggested  to 
us  by  our  process  for  removing  air  from  the  porous  walls  of  osmotic  cells, 
i.  e.,  the  method  of  "electrical  endosmose."  A  large  porous  pot  (usually  a 
flower  pot)  is  placed  in  a  larger,  water-tight  vessel  of  any  suitable  mate- 
rial. The  clay  (generally  in  the  form  of  a  thick  porridge)  is  poured 
into  the  former.  Two  electrodes  are  inserted,  the  anode  into  the  con- 
tents of  the  porous  pot,  and  the  cathode  into  the  water  which  quickly 


8 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


collects  in  the  outer  vessel.  As  the  level  of  the  contents  of  the  pot 
recedes,  more  clay  is  added  until  no  more  can  be  introduced. 
In  the  meantime,  the  level  of  the  water  in 
the  outer  receptacle  is  kept,  by  means  of  an 
automatic  siphon,  just  high  enough  to  permit 
the  complete  submersion  of  the  cathode. 
When  the  clay  becomes  so  far  dried  that  the 
mass  begins  to  crack  at  the  top,  it  is  packed 
down  with  a  heavy  pestle.  In  this  way  the 
excess  of  water  can  be  separated  from  the 
clay  much  more  rapidly  and  even  more  com- 
pletely than  by  filtration  under  diminished 
pressure.  The  method  can  also  be  applied 
with  advantage  to  the  separation  of  water 
from,  and  even  to  the  washing  of,  other  solids 
which,  like  clay,  are  filtered  with  difficulty. 
The  voltage  employed  is  110  or  120. 


THE  FORMATION  OF  THE  CYLINDERS. 

A  section  of  one  of  the  steel  presses  in  which 
the  clay  is  formed  into  cylinders  is  shown  in 
Figure  1  A.  The  barrel  (1)  is  slightly  tapered 
internally,  the  diameter  at  the  bottom  being 
0.005  inch  greater  than  at  the  top,  in  order  to 
insure  the  ready  release  of  the  clay  cylinder 
when  it  is  to  be  pushed  out  of  the  lower  end 
of  the  press.  It  is  threaded  at  both  ends  to 
receive  the  caps  (2  and  3).  The  cap  at  the 
lower  end  (2)  is  bored  to  permit  the  escape 
of  the  water  which  is  squeezed  out  of  the  clay. 
The  cap  at  the  upper  end  (3)  is  bored  and 
threaded  internally  to  receive  the  hollow  plug 
(4).  The  steel  disks  (5  and  6)  are  also  bored 
to  facilitate  the  escape  of  water.  The  disks 
(7  and  8)  are  of  porous  hard-burned  clay  or  of 
asbestus. 

The  upper  steel  disk  (6)  is  less  simple  than 
it  appears  in  the  figure.  In  reality  it  consists 
of  two  grooved  disks  separated  by  hardened 
steel  balls  (bicycle  balls),  as  shown  in  Figure 
1  B.  The  upper  half  turns  readily  with  the 
plug  (4),  while  the  lower  half  remains  sta- 
tionary, thus  preventing  any  twisting  of  the 
clay  beneath.  If  a  single  disk  is  used,  the 
clay  is  twisted  in  a  direction  the  reverse  of 


A 


B 


FIG.  1. 

A.  Steel  press  for  clays.  (1)  Bar- 
rel; (2)  lower  cap;  (3)  upper  cap; 
(4)  plunger ;  (5)  and  (6)  steel  disk ; 
(7)  and  (8)  porous  clay  disk. 

B.  Ball-bearing   disk,   used    in 
place  of  (6)  to  prevent  twisting 
of  clay. 

that  of  the  screw,  and 


CELLS   AND   MANOMETER   ATTACHMENTS.  9 

the  cell,  when  burned,  exhibits  upon  its  exterior  surface  a  series  of 
spirally  arranged  elevations  or  depressions,  as  if  the  shrinkage  of  the 
clay  in  baking  had  not  been  entirely  uniform.  Considerable  difficulty 
was  experienced  at  first  in  securing  the  correct  temper  for  the  grooved 
disks,  which,  of  course,  should  be  equal  to,  but  not  much  higher  than, 
that  of  the  steel  balls  which  separate  them.  If  the  disks  are  insuffi- 
ciently tempered,  they  are  badly  lacerated  by  the  balls.  On  the  other 
hand,  if  they  are  made  too  hard,  they  frequently  crack  under  the  great 
pressure  to  which  the  clay  is  subjected. 

In  order  that  the  diameter  of  the  clay  cylinders  may  be  varied,  the 
barrel  of  the  press  (Figure  1  A)  is  made  quite  wide  (2.5  inches  internally) 
and  is  provided  with  a  series  of  steel  "sleeves"  of  various  smaller  bores — 
which  may  be  inserted.  Each  sleeve  requires,  of  course,  its  own  set  of 
disks  (Figure  1 A  5,  6,  7,  and  8,  and  Figure  1  B).  The  length  of  the  cyl- 
inder is  regulated  by  the  number  and  thickness  of  the  disks  (5),  which 
are  placed  in  the  bottom  of  the  press  before  introducing  the  clay. 

The  two  clays,  prepared  as  previously  described,  mingle  readily  in  all 
proportions,  giving  products  which,  when  baked,  are  uniform  in  respect 
to  texture  and  strength.  It  was  found  that  all  the  requirements  of  the 
situation  are  best  met  by  mixing  them  in  about  equal  proportions  by 
weight.  The  process  of  mixing  is  as  follows:  (1)  Equal  weights  of  the 
air-dried  and  pulverized  clays  are  mingled  and  repeatedly  sifted;  (2) 
the  mixture  is  churned  with  water  for  several  hours,  after  which  (3)  it 
is  bolted — without  unnecessary  interruption  of  the  churning  process — 
through  Nos.  14  and  16  bolting-cloth;  (4)  the  material  is  allowed  to 
subside,  and  the  supernatant  water  is  removed  by  means  of  a  siphon; 
(5)  the  major  portion  of  the  large  excess  of  water  still  remaining  with 
the  clay  is  removed  by  draining  upon  a  filter  of  bolting-cloth  resting 
upon  one  of  paper,  or  by  the  "endosmose"  method  already  described ;  (6) 
finally  the  material  is  extensively  kneaded  and  mixed  upon  a  plate- 
glass  surface  until,  through  evaporation  of  the  water,  the  "putty"  has 
attained  the  consistency  which  experience  has  shown  to  be  best  suited 
to  pressing. 

The  putty,  which  must  never  be  touched  without  first  covering  the 
hands  with  rubber  gloves,  is  "tamped"  down  in  the  press  with  a  steel 
plunger  which  has  been  cleansed  with  ether. 

The  device  for  compressing  the  clay  is  shown  in  Figure  2,  without  the 
framework  which  holds  the  various  parts  in  their  places.  The  press 
(1)  containing  the  clay  is  secured  between  two  flat  bars  of  steel,  a  por- 
tion (2)  of  one  of  which  is  seen  in  the  figure.  The  lower  end  of  the 
vertical  shaft  (3)  is  square  in  form,  like  the  upper  end  of  the  plunger  of 
the  press  (Figure  1  A),  and  the  collar  (4),  which  joins  the  two,  has  a 
square  hole  of  the  same  diameter  passing  through  it.  A  portion  of  two 
of  the  timbers  of  the  framework  is  shown  in  the  figure  (5  and  6). 
Through  these,  the  shaft  (3)  slides  freely  up  and  down,  except  so  far  as 
its  motion  is  limited  by  the  set  collar  (7).  The  large  wooden  drum  (8) 


10 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


is  firmly  attached  to  the  shaft,  and  around  it  is  wound  the  steel-wire 
cable  (9).  The  loose  iron  pulleys  (10,  11,  and  12)  serve  to  guide  the 
cable.  The  large  pulley  (12)  is  situated  in  the  attic  of  the  laboratory. 
The  cable,  after  leaving  the  horizontal  pulley  (11),  ascends  vertically 
through  the  ceiling  of  the  room  and  passes  over  the  attic  pulley  (12). 
The  descending  end  of  the  cable  is  attached  to  a  heavy  iron  rod  (13), 
upon  which  may  be  loaded  any  required  number  of  cast-iron  weights 
(14  and  15).  At  the  floor,  the  weight  (consisting  of  13,  14,  and  15) 
enters  a  vertical  shaft  more  than  50  feet  in  depth.  The  detachable 
wrench  (16),  which  is  provided  with  extensions,  is  employed  in  raising 
the  weight  and  coiling  the  cable  about  the  drum. 


FIG.  2. — Apparatus  for  pressing  clays. 

(1)  Steel  press  (see  Fig.  1  A);  (2)  steel  frame  for  holding  press;  (3)  movable  steel  shaft;  (4)  collar 
joining  (1)  and  (3);  (5)  and  (6)  parts  of  wooden  framework;  (7)  set  collar  to  limit  vertical 
motion  of  shaft;  (8)  drum  for  coiling  cable;  (9)  steel  cable;  (10)  vertical  loose  guide  pulley  for 
cable;  (11)  horizontal  loose  guide  pulley  for  cable;  (12)  loose  cable  pulley  in  attic;  (13) 
saddle  for  weights;  (14)  and  (15)  cast-iron  weights. 

After  filling  the  press,  and  before  placing  it  in  the  position  shown  in 
Figure  2,  it  is  put  into  a  vise  and  considerable  of  the  water  is  forced  out 
by  use  of  the  wrench  (16),  which  is  given  a  turn  from  time  to  time  as  the 


CELLS   AND    MANOMETER   ATTACHMENTS. 


11 


escape  of  water  makes  further  compression  of  the  clay  possible.  When 
no  more  water  can  be  forced  out,  the  press  is  transferred  to  its  place  in 
Figure  2.  The  amount  of  weight  to  be  applied  and  the  duration  of  the 
period  of  pressing  are  judged  entirely  by  previous  observations  on  the 
fitness  of  the  products  for  cutting  purposes.  In  general,  the  weight 
employed  is  that  which  will  give  a  calculated  pressure  of  20  tons  upon 
each  square  inch  of  the  surface  of  the  clay  cylinder  after  an  allowance  of 
one-third  for  loss  by  friction.  The  first  descent  of  the  weight  (53  feet) 
is  usually  accomplished  in  about  2  hours  and  the  second  in  about  16 
hours.  Ordinarily  the  pressing  is  discontinued  at  the  end  of  the  second 
excursion  of  the  weight.  Equivalent  results  can  be  secured  by  lighter 
weights  and  longer  pressing  or  by  heavier  weights  and  shorter  pressing. 
The  object  to  be  attained  is,  of  course,  that  condition  of  the  clay  which 
will  enable  one  to  cut  a  perfect  cell  from  the  pressed  cylinder,  and  for 
this  purpose  the  clay  must  be  neither  too  wet  nor  too  dry. 

When  the  cylinder  is  to  be  removed,  the  press  is  again  placed  in  the 
vise,  the  upper  cap  (Figure  1  A)  is  removed  and  an  additional  disk  is 
introduced.  On  replacing  the  upper  cap 
and  removing  the  lower  one,  and  giving 
the  plunger  (Figure  1  A,  4)  a  slight  turn, 
the  cylinder  is  effectively  released,  and — 
owing  to  the  tapered  form  of  the  barrel 
(Figure  1  A) — uninjured.  The  form  of 
the  cylinder  is  shown  in  Figure  3  A. 

THE  CUTTING  OF  THE  CELLS. 

One  of  the  commoner  forms  of  the 
finished  cell  as  it  is  turned  out  of  the 
cylinder  (Figure  3  A}  is  shown  in  Figure 
3  B.  Other  forms  will  be  represented 
when  the  attachment  of  the  manometer 
to  the  cell  is  discussed. 

The  cutting  of  the  cells  from  the  cyl- 
inders is  an  exceedingly  critical  opera- 
tion, which  requires  experience  and  well- 
developed  mechanical  instincts.  Very 
few,  even  of  those  who  have  had  mechanical  training,  ever  succeed 
in  the  undertaking.  The  explanation  of  so  many  failures  is  very 
simple:  The  cell  wall  must  not  be  weakened  at  any  point  by  the  pres- 
sure of  the  cutting  tools;  because,  when  the  cell  is  baked,  the  shrinking 
material  (the  shrinkage  is  between  7  and  8  per  cent)  necessarily  draws 
away  from  the  regions  of  relative  weakness  toward  those  where  the 
cohesion  of  the  particles  is  stronger  and  cracks  are  developed.  It  is 
by  no  means  necessary  that  the  damage  done  by  irregular  or  excessive 
pressure  from  the  cutting  tools  should  be  apparent  in  the  finished 


FIG.  3. 

A.  Clay  cylinder  after  pressing. 

B.  Clay  cell  after  shaping  the  cylinder 

(Fig.  3  A)  on  the  lathe. 


12 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


product.  In  fact,  it  is  rarely  discovered  until  the  cells  are  taken  from 
the  kiln.  At  first,  the  failures  from  the  cause  alluded  to  were  over  90 
per  cent.  At  the  present  time,  about  10  per  cent  of  the  cells  develop 
cracks  while  in  the  kiln.  The  improvement  has  been  due  in  a  large 
measure  to  improvements  in  the  cutting  tools  and  to  the  increased  atten- 
tion which  has  been  given  to  keeping  them 
in  good  order.  It  was  found  impossible  to 
succeed  with  the  usual  lathe  cutting  tools, 
and  others  with  new  forms  of  cutting  edge 
were  designed.  One  of  the  more  important 
of  these  is  shown  in  Figure  4.  It  is  the  tool 
with  which  all  the  boring  and  nearly  all  the 
inside  work  are  done.  It  will  be  seen  that 
the  tool  cuts  only  in  the  longitudinal  direc- 
tion of  the  cylinder,  bringing  no  pressure 
upon  the  wall  of  the  cell  in  a  transverse 
direction.  The  same  principle  is  employed 
in  fashioning  the  tools  with  which  the  out- 
side work  is  done.  But  however  good  the 
design  of  the  tool  may  be,  failure  is  bound 
to  attend  its  use  in  this  work  unless  it  is 
ground  in  accordance  with  correct  princi- 
ples, i.  e.,  with  the  proper  "clearance"  and 
is  always  maintained  in  a  sharp  condition. 

A  multitude  of  details  relating  to  methods 
of  mounting,  speeds  of  cutting,  etc.,  all  of 
which  are  of  importance  to  the  operator,  but 
of  little  interest  to  others,  are  omitted,  and 
only  one  instance  of  the  many  precautions 
which  it  is  necessary  to  observe  will  be  men- 
tioned— the  fact,  namely,  that,  when  the  lathe 
has  once  been  started,  it  must  not  be  stopped 
until  the  cell  is  finished,  owing  to  the  danger 
of  "sagging" 

THE  BURNING  AND  GLAZING  OF  THE  CELLS. 

The  experimental  work  in  the  baking  of 
clays  was  done  for  several  years  either  in  a  FlG-  4.— Different  views  of  special 

o,  ,  .,  .  -,       .    •      f  rrti  to°l  f°r  cutting  cell   (Fig.  3B) 

Seger  kiln  or  in  an  electric  furnace.      Ihe     from  cylinder  (Fig.  3  4). 
electric  furnace  which  was  first  employed  was 

one  in  which  the  platinum  wires  were  woven  through  holes  in  the  walls 
and  bottom  of  the  furnace,  so  that  the  heat  generated  in  the  wires 
must  penetrate  a  considerable  thickness  of  clay  before  reaching  the 
space  to  be  heated.  With  such  an  arrangement  a  very  long  time 
is  required  to  obtain,  with  a  given  current,  the  temperature  which 


CELLS   AND   MANOMETER   ATTACHMENTS.  13 

that  current  will  eventually  maintain  in  the  furnace.  In  the  case 
of  the  instrument  here  mentioned,  from  7  to  9  hours  were  required 
for  that  purpose.  A  close  regulation  of  the  temperature  was  there- 
fore impossible.  Another  objection  to  this  furnace  was  its  waste- 
fulness. At  1250°  the  consumption  of  electrical  energy  was  equivalent 
to  1200  watts.  The  furnace  was  improved  to  some  extent  by  certain 
modifications  which  were  introduced,  but  not  sufficiently  to  justify  its 
continued  use.  It  was  therefore  abandoned  for  one  of  our  own  con- 
struction,* in  which  the  wires  were  all  exposed  in  the  space  to  be  heated. 
The  saving  in  electricity  thereby  effected  was  over  50  per  cent.  The 
new  furnace  is  shown  in  Figures  5  A,  5  B,  5  C,  and  6.  It  will  be  seen  to 
consist  (Figure  5  A)  of  platinum  wires  threaded  through  three  clay  rings 
(a,  6,  and  c),  which  are  held  apart  by  three  platinum  rods.  The  rods 
expand  in  the  same  degree  as  the  wires,  and  thus  keep  the  latter  taut, 
whatever  may  be  the  temperature  of  the  furnace.  Otherwise  the  wires 
would  "buckle"  and  short  circuit  at  high  temperatures.  The  wires  are 
in  two  pieces  of  equal  length,  so  that  they  may  be  placed  in  series  or  in 
parallel,  according  to  the  amount  of  current  which  it  is  desired  to  use. 
Figure  5B  shows  the  furnace  in  place  in  the  innermost  (d)  of  the  clay 
cylinders  which  surround  it  when  in  use.  The  cover  (e),  the  bottom 
(/),  and  the  truncated  cones  (g)  on  which  the  furnace  rests  are  also 
represented  in  the  figure.  Figure  5  C  represents  the  outer  clay  cylinder 
and  its  various  accessories.  In  Figure  6  all  the  parts,  lettered  as  in 
Figures  5  A ,  5  B,  and  5  C,  are  assembled  as  a  crucible  furnace.  The  outer 
covering  (ra)  is  a  sheet-iron  cylinder,  which  is  covered,  internally  and 
externally,  with  asbestus  paper.  The  purpose  of  the  remaining  parts 
(n,  o,  p,  q,  r,  and  s)  is  obvious  without  explanation. 

The  electric  kilns  (of  which  three  were  usually  in  operation)  were  all 
calibrated  by  means  of  a  Le  Chatelier  pyrometer.  They  thus  became, 
in  themselves,  resistance  pyrometers,  the  temperature  of  which  could  be 
easily  ascertained  at  all  times.  The  electric  kilns  answered  well  the 
purpose  for  which  they  were  constructed  up  to  about  1200°,  i.  e.,  to  a 
temperature  at  which  platinum  begins  sensibly  to  volatilize  in  an 
atmosphere  containing  oxygen.  At  higher  temperatures,  the  loss  of 
platinum  was  sufficient  to  make  an  occasional  recalibration  necessary. 

The  best  results  were  obtained  at  about  1300°,  i.  e.,  between  the 
melting-points  of  Seger  cones  Nos.  8  and  9.  Having  ascertained  the 
most  advantageous  temperature  for  burning  the  cells,  there  was  no 
longer  any  good  reason  for  baking  them  in  the  laboratory  rather  than 
at  the  pottery.  Fortunately,  at  the  opportune  time,  we  were  offered 
the  free  use  of  the  kilns  of  the  Chesapeake  Pottery  Company  by  the 
late  president  of  that  concern,  Mr.  D.  F.  Haynes.  A  similar  courtesy 
was  also  extended  to  us  by  the  Bennett  Pottery  Company.  At  the 

*Amer.  Chem.  Journal,  xxxn,  93. 


14 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


FIG.  5. 


C 


A.  Electric  kiln  for  baking  cells,     (a),  (6),  and  (c)  perforated  clay  rings,  held  in  place  by  three 

platinum  rods  which  prevent  the  platinum  wires  from  "  buckling"  when  hot. 

B.  Inner  covering  for  electric  kiln,     (d)  Clay  cylinder;  (e)  cover;    (/)  bottom;  (g)    truncated 

clay  cones. 

C.  Outer  covering  for  electric  kiln,     (h)  Clay  cylinder;  (f)  bottom;  (fc)  truncated  clay  cones. 


FIG.  6. — Electric  kiln  arranged  as  crucible  furnace. 

(a)  to  (k)  the  same  as  in  Figs.  5  A,  5  B,  5  C;  (I)  Le  Chatelier  pyrometer;  (m)  sheet-iron 
cylinder  covered  with  asbestus;  (n),  (o),  and  (p)  parts  of  base;  (q),  (r),  and  (s)  parts  of  elec- 
trical connections;  (0  rest  for  crucible. 


CELLS   AND   MANOMETER   ATTACHMENTS.  15 

potteries,  we  could  neither  control  nor  know  with  certainty  the  tem- 
perature of  any  part  of  the  kilns,  but  the  places  in  them  where  the  best 
results  are  most  frequently  obtained  were  easily  found,  and  since  then 
all  of  the  cells  have  been  burned  at  the  potteries. 

It  has  been  stated  elsewhere  that,  in  the  endeavor  to  produce  the  cor- 
rect texture  of  cell  wall,  we  made  a  study  of  thin  sections,  both  of  the 
potters'  cells  and  of  our  own.  In  the  course  of  this  work,  a  considerable 
number  of  photographs  were  accumulated,  6  of  which  are  here  repro- 
duced. Three  of  them  (Plate  1,  a,  c,  and  e)  are  from  potters'  cells,  and 
three  (b,  d,  and  /)  are  from  the  first  cells  made  by  us  which  proved  them- 
selves well  suited  to  the  measurement  of  osmotic  pressure.  It  will  be 
noted  that  the  texture  of  the  cells  made  in  the  laboratory  is  incom- 
parably finer  than  that  of  the  potters'  products.  But  we  were  con- 
vinced, after  nearly  five  years  of  laborious  investigation,  that  just  this 
excessive  fineness  of  texture  is  absolutely  indispensable  to  the  correct 
measurement  of  osmotic  pressure.  It  is  necessary,  in  the  first  place,  in 
order  that  the  membrane  may  be  deposited  exclusively  upon  the  inner 
surface  of  the  cell  wall.  It  is  not  meant  by  this  statement  that  no  part 
of  the  membrane  is  to  be  found  within  the  pores.  On  the  contrary,  all 
good  membranes  are  found,  on  microscopic  examination,  to  be  firmly 
rooted  in  the  mouths  of  the  pores  which  open  behind  it.  It  is  this  feat- 
ure, in  fact,  which  makes  membranes  produced  by  the  electrolytic 
method  so  much  superior  to  those  which  were  made  by  the  older  pro- 
cess. Fineness  of  texture  is  also  necessary  in  order  to  give  the  mem- 
brane a  backing  which  will  enable  it  to  withstand  pressure.  If  it  is 
more  open  than  that  shown  in  Plate  1,  b,  d,  and  /,  the  membrane  is 
deposited,  at  least  partially,  within  the  cell  wall,  and  it  breaks  under 
moderate  pressure. 

It  is  desirable  to  explain  the  numerous  black  specks  seen  in  Plate  1, 
b,  d,  and  /.  They  are  particles  of  the  emery  used  in  grinding  the  sec- 
tions, and  no  part  of  what  the  photographs  are  intended  to  show. 

The  exact  extent  to  which  the  sections  here  represented  were  magni- 
fied can  not  now  be  stated,  the  original  records  having  been  mislaid  or 
lost,  but  it  is  believed  to  have  been  125  diameters. 

The  question  naturally  arises,  whether  it  is  possible  to  make  the  text- 
ure of  a  cell  wall  too  close,  provided,  of  course,  it  still  remains  porous  to 
some  extent.  The  effective  area  of  a  membrane  is  equal  to  the  aggre- 
gate area  of  the  pore-openings  upon  the  interior  surface  of  the  cell  wall, 
and  it  has  been  found  quite  possible,  by  hard  burning,  so  to  diminish  this 
area  of  membrane  as  to  make  the  passage  of  solvent  into  or  out  of  the 
cell  intolerably  slow.  Some  evidence  has  also  been  gathered  to  show  that 
the  reduction  in  the  size  of  the  pores  may  be  carried  to  such  an  extent 
that  the  membrane  no  longer  roots  itself  firmly  into  them.  This  is  the 
explanation  given  to  the  formation  of  the  detachable  membranes  which 
are  sometimes  deposited  in  very  hard-burned  cells.  It  is  imagined  that, 


16       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

in  such  cases,  the  ferrocyanogen  ions  do  not  get  far  enough  into  the  pores 
before  meeting  those  of  copper  coming  from  the  opposite  direction — in 
other  words,  that  the  membrane  is  formed  at  or  without  rather  than 
within  the  mouths  of  the  pores. 

After  baking  the  cells  and  before  glazing  them,  they  are  mounted  on 
the  lathe  and  ground  under  the  shoulder  with  a  high-speed  carborundum 
wheel,  to  fit  the  brass  rings  with  which  the  manometers  are  fastened 
in  their  places.  The  necks  are  also  ground  to  the  exact  taper  of  the 
cones  upon  the  ends  of  the  manometers. 

The  finding  of  a  suitable  glaze  for  the  upper  half  of  the  cells  was  a 
matter  of  considerable  difficulty.  As  might  have  been  expected,  the 
expansion  coefficient  of  products  made  as  these  cells  are  is  very  different 
from  that  of  any  of  the  potters'  wares.  Hence  none  of  the  glazes  which 
are  used  by  the  potters  would  meet  the  requirements  of  the  situation. 
All  such  glazes  were  found  to  "craze"  badly  upon  the  biscuit.  An 
attempt  was  made  to  glaze  with  feldspar,  but  with  poor  success.  A 
wholly  suitable  glazing  material  was  finally  obtained  by  adding  silica 
and  feldspar  to  one  of  the  glazes  which  are  used  by  the  potters  upon  the 
better  grades  of  their  white  tableware.  The  earlier  experiments  in 
glazing  were  carried  out  in  a  Seger  gas  kiln,  but  at  the  present  time  the 
glazing,  as  well  as  the  baking  of  the  cells,  is  done  at  the  potteries. 

There  is  one  objection  to  glazing  the  cells  to  which  attention  should 
be  called.  They  are  glazed,  inside  and  outside,  from  the  middle 
upward,  leaving  the  lower  half  of  the  cells  porous.  The  whole  interior 
of  the  cell  is  therefore  protected  at  all  times,  either  by  the  glaze  or 
the  membrane,  so  that  no  material  in  solution  can  diffuse  into  the  wall 
from  the  inside.  On  the  outside,  the  case  is  different.  There  it  is  quite 
possible  for  the  dissolved  substances  to  diffuse  upward  and  accumulate 
between  the  inner  and  outer  glazed  surfaces.  If  these  were  allowed  to 
remain  and  should  afterwards  diffuse  downward  and  distribute  them- 
selves about  the  membrane,  the  pressure  measured  would  not  be  that  of 
the  solution  within  the  cell,  but  rather  the  difference  between  the  pres- 
sures of  the  solutions  on  the  opposite  sides  of  the  membrane.  It  is  not 
believed  that  the  results  to  be  reported  in  later  chapters  have  been  at 
all  vitiated  by  this  possible  source  of  error;  because  it  has  always  been 
necessary,  in  order  to  maintain  unimpaired  the  colloidal  state  of  the 
membrane,  to  soak  the  cell  for  considerable  intervals  in  pure  water 
between  any  two  successive  experiments.  Nevertheless,  it  seemed  de- 
sirable to  produce  a  cell,  the  upper  half  of  which  has  the  non-permeable 
character  of  porcelain,  while  the  lower  half  remains  porous.  The  diffi- 
culty is,  of  course,  to  prepare  clay  mixtures  for  the  two  parts  of  the  cell 
which  shall  maintain  identical  expansion  coefficients  throughout  the 
whole  of  the  baking  and  cooling  periods — at  least  at  all  points  of  union 
between  them.  Otherwise  cracks  or  a  condition  of  weakness  must 
develop  at  the  junction  of  the  two  clays. 


MORSE 


FIGS,  a,  c,  and  e,  thin  sections  taken  from  potter's  cells. 

FIGS,  o,  d,  and  f,  thin  sections  taken  from  cells  made  in  the  laboratory. 


CELLS   AND   MANOMETER   ATTACHMENTS.  17 

Occasional  experiments  with  a  view  to  producing  a  half-porcelain, 
half-porous  cell  have  been  carried  out  along  two  lines :  first,  by  so  mix- 
ing the  two  kinds  of  clays  that  for  a  certain  distance  from  the  center, 
upward  and  downward,  each  kind  would  disappear  gradually;  second, 
by  mixing  some  of  the  glazing  material  with  the  clay  which  was  to  form 
the  upper  half  of  the  cell.  The  results  have  been  encouraging,  though 
up  to  the  present  time  not  wholly  satisfactory. 

THE  MANOMETER  ATTACHMENTS  OF  THE  CELLS. 

Great  difficulty  has  been  experienced  in  devising  suitable  arrange- 
ments for  attaching  the  manometers  to  the  cells.  The  problem  is  less 
simple  than  it  might  appear  to  be  at  first  sight.  Three  things  must  be 
provided  for  in  any  workable  device  for  closing  the  cell:  (1)  a  junction 
which  will  not  leak  at  high  pressure;  (2)  means  of  adjusting,  at  will,  the 
pressure  in  the  cell  (this  is  especially  necessary  when  manometers  of 
large  capacity  are  used) ;  and  (3)  an  arrangement  so  simple  in  manipu- 
lation that  the  cell  can  be  filled  and  closed  and  the  proper  initial  pres- 
sure established  in  a  fraction  of  a  minute.  Several  schemes  have  been 
employed  for  joining  the  cell  to  the  manometer,  all  of  which,  with  two 
exceptions,  are  still  in  use.  Some  of  the  arrangements  which  worked 
satisfactorily  at  moderate  temperatures  failed  utterly  at  high  temper- 
atures. 

The  first  crude  experiments*  were  made  with  cells  into  which  rubber 
stoppers — carrying  manometers — were  thrust  and  fastened  in  place  as 
well  as  might  be  with  wire.  The  highest  pressure  obtained  by  such 
means  was  only  4.5  atmospheres.  The  manometers  were  pushed  out  of 
the  cells  and,  owing  to  the  tendency  of  rubber  to  flow  into  regions  of  less 
pressure,  the  stoppers  were  badly  distorted.  The  earlier  experiments, 
however,  were  only  qualitative.  They  were  made  in  order  to  test  the 
membrane  rather  than  with  a  view  to  measuring  osmotic  pressure. 
Other  qualitative  experiments  were  carried  out  laterf  with  somewhat 
improved  apparatus,  but  the  earliest  successful  attempts^  to  measure 
osmotic  pressure  were  made  in  the  apparatus  shown  in  Figure  7. 

The  porous  cell  (A),  which  is  unglazed,  is  ground  out  internally  to  a 
distance  from  the  open  end  which  is  a  little  over  one-third  its  depth, 
until  the  shoulder  formed  at  the  bottom  of  the  ground  part  extends 
entirely  around  the  cell  and  is  of  sufficient  width  to  afford  an  ample  sup- 
port for  the  soapstone  ring  (6).  Afterwards  two  channels,  one  of  which 
is  designated  in  the  figure  by  the  letter  a,  are  cut  into  the  wall  to  pre- 
vent the  dislodgment  of  the  cement  under  pressure.  The  glass  tube 
(JB),  which  connects  the  cell  with  the  manometer,  is  enlarged  in  two 
places  (c  and  d)  to  prevent  its  displacement,  and  is  contracted  at  the  top 
to  give  it  a  better  grip  upon  the  rubber  stopper  (e).  The  soapstone 
ring  (6)  is  accurately  fitted  to  its  place  in  the  cell  and  also  to  the  glass 

*Amer.  Chem.  Journal,  xxvi,  80.         \Ibid.,  xxvin,  1.         %Ibid.,  xxxiv,  1. 


18       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

tube  (#),  the  end  of  the  latter  having  been  ground  to  a  perfectly  cir- 
cular form.  The  lower  end  of  the  glass  tube  is  beveled  inward  to  pre- 
vent the  lodgment  of  air.  The  purposes  of  the  brass  parts  (g,  h,  and  o) 
are  obvious  without  explanation.  The  tube  (B~)  is  set  in  the  brass 
piece  (o)  and  in  the  cell  (A)  with  litharge-glycerine  cement.  But 
before  proceeding  to  the  latter  operation,  the  glass  tube,  with  the  soap- 
stone  ring  in  place,  is  inverted,  and  any  space  which  is  left  between 
them  is  filled  with  molten  shellac.  The  tube  and  ring  are  then  heated 
in  an  air  bath  until  the  shellac  remains  solid  at  100°.  The  cement 
employed  to  fix  the  tube  and  the  ring  (B  and  6)  in  their  places  in  the 
cell  (A),  and  also  the  shellac  used  to  join  b  to  B,  must  be  effectually 
protected  from  any  contact  with  the  solution  in  the  cell  or  the  water 
outside  of  it.  For  this  purpose,  the  lower  end  of  the  glass  tube,  the 
soapstone  ring,  and  the  whole  of  the  ground  surface  within  the  cell  are 
repeatedly  painted  with  a  dilute  solution  of  rubber.  When  a  covering 
of  sufficient  thickness  has  been  obtained,  the  soapstone  ring — which  is 
now  firmly  attached  to  the  glass  tube — is  crowded  into  its  place  on  the 
"shoulder."  The  operation  is  liable  to  lacerate  more  or  less  the  rubber 
covering  of  the  cell  wall.  To  repair  any  damage  of  this  kind,  and  also 
to  insure  a  tight  joint  between  the  clay  wall  and  the  soapstone  ring,  the 
whole  cavity  above  the  latter  is  again  painted  with  the  rubber  solution. 
The  apparatus  is  then  placed  in  an  air-bath  and  maintained  at  100° 
until  the  rubber  becomes  quite  hard  but  not  brittle.  Finally  the  space 
between  the  glass  tube  and  the  cell  wall  is  filled  with  the  usual  mixture 
of  litharge  and  glycerine.  The  lower  end  of  the  manometer  is  enlarged 
0)  t°  prevent  its  being  pushed  upward  through  the  stopper  (&).  The 
purposes  of  the  cork  (1)  and  of  the  bottle  (ra)  do  not  require  explanation. 
A  special  instrument,  which  came  to  be  known  as  the  "fang,"  is 
required  both  to  close  and  to  open  the  cell.  It  is  shown  in  Figure  8.  It 
consists  of  a  round,  slender,  and  tapered  piece  of  steel,  one  end  of  which 
has  been  furrowed  out  upon  one  side  and  bent  into  the  curved  form  seen 
in  the  figure.  It  was  usually  made  from  a  small  round  file  from  which 
the  temper  had  been  drawn.  The  "fang"  is  inserted  between  the 
rubber  and  the  glass  tube  at  e,  to  permit  the  escape,  through  the  furrow, 
of  the  excess  of  liquid  when  the  cell  is  closed,  and  again  to  provide  for 
the  entrance  of  air  when  the  cell  is  opened.  It  is  likewise  of  great  assist- 
ance, when  manipulated  as  a  lever,  in  introducing  and  removing  the 
stopper  through  the  narrow  mouth  of  the  tube.  The  stopper  from  e 
upward  is  tightly  wound  with  shoemakers'  waxed  thread  to  prevent  the 


FIG.  8. — The  "fang"  for  the  introduction  and  removal  of  the  rubber  stoppers  (k,  Fig.  7). 


CELLS   AND   MANOMETER   ATTACHMENTS. 


19 


FIQ.  7. — First  form  of  complete  cell. 
(A)  Porous  cell;  (B)  glass  tube;  (C)  manometer; 
(a)  groove  cut  in  cell;  (6)  soapstone  ring; 
(c)  and  (d)  enlargements  in  glass  tube,  to  pre- 
vent slipping;  (e)  contraction  at  upper  end  of 
glass  tube;  (/)  and  (n)  litharge-glycerine 
cement;  (g)  brass  collar;  (A)  brass  nut;  (t) 
concave  brass  piece;  (j)  enlargement  on  end 
of  manometer;  (k)  rubber  stopper;  (/)  cork; 
(TO)  glass  bottle;  (o)  brass  piece. 


Fia.  9. — Second  form  of  complete  cell. 
(1)  Brass  collar;  (2)  brass  nut;  (3)  lead 
washer;  (4)  urasscone;  (5)  manometer; 
(6)  hollow  needle;  (7)  fusible  metal; 
(8)  brass  piece  to  which  the  needle 
is  brazed;  (9)  steel  screw  threaded  into 
(8);  (10)  packing;  (11)  fusible  metal 
covering  exposed  part  of  needle;  (12) 
rubber  tubing;  (13)  and  (14)  windings 
with  twisted  shoemakers'  thread. 


20        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

rubber  from  oozing  out  of  the  glass  tube.  The  initial  pressure  in  the 
cell  is  adjusted  by  means  of  the  nut  (h)  and  the  collar  (g). 

The  arrangement  described  above  was  employed  for  the  measurement 
of  osmotic  pressure  from  1905,  when  the  first  good  cells  were  obtained, 
until  1908,  when,  for  reasons  which  will  be  stated,  but  not  fully  discussed 
until  later,  it  was  abandoned  for  the  apparatus  which  is  shown  in 
Figure  9. 

The  principal  objections  to  the  first  apparatus  employed  for  quanti- 
tative purposes  (stated  in  the  order  of  their  importance)  were: 

1.  The  length  of  time  required  to  close  and  open  the  cell.    During 
both  periods,  the  contents  of  the  cell,  being  necessarily  under  less  than 
maximum  pressure,  became  diluted  by  the  water  which  entered  through 
the  membrane. 

2.  The  difficulty  of  the  manipulation  required  properly  to  introduce 
and  remove  the  rubber  stopper  without  injury  to  the  manometer. 

3.  The  frequent  bursting  of  the  glass  tube  (B),  which  was  usually 
attended  by  the  total  loss  of  the  cell  (A) ;  since,  as  a  rule,  the  membrane 
was  ruined  by  the  measures  taken  to  replace  a  broken  tube. 

The  apparatus  represented  in  Figure  9*  is  a  decided  improvement  on 
that  shown  in  Figure  7.  In  it  the  difficulties  enumerated  above  are 
obviated,  though,  as  will  be  seen  later,  it  has  certain  defects  of  its  own. 
The  function  of  the  brass  collar  (1)  and  of  the  brass  nut  (2)  will  be 
readily  understood  without  explanation.  The  form  of  these  pieces  has 
varied  but  little  from  the  beginning.  The  lead  ring  (3)  separates  the 
shoulder  of  the  cell  from  the  flange  of  the  brass  collar  and  serves  to  pro- 
tect the  glaze  upon  the  former.  A  ring  of  softer  material,  e.  g.,  leather, 
can  not  be  used  for  the  purpose,  since  any  upward  movement  of  the 
collar,  due  to  diminishing  thickness  of  the  ring  under  pressure,  leads  to 
an  increase  in  the  capacity  of  the  cell  and  a  dilution  of  the  solution.  In 
other  words,  the  ring  (3)  must  be  of  fairly  rigid  material.  The  brass 
cone  (4)  has  two  holes  passing  entirely  through  it,  one  for  the  mano- 
meter tube  (5)  and  the  other  for  the  hollow  needle  (6),  both  of  which 
(the  manometer  and  the  needle)  are  securely  fastened  in  the  cone  by 
some  fusible  metal  (Wood's,  Rose's,  or  Babbit's).  The  holes  through 
the  cone  are  bored  slightly  larger  than  the  tubes  which  are  to  occupy 
them,  in  order  that  the  molten  metal  may  flow  down  and  completely  fill 
the  space  between  the  latter  and  the  walls  of  the  former.  In  this  way 
the  tubes  are  more  firmly  fixed  in  their  places  and  all  danger  of  leakage 
upward  through  the  cone  is  avoided.  The  hollow  tube  (6) — the 
needle — is  nickel-plated  and  is  brazed  into  the  brass  piece  (8),  which  is 
bored  out  and  threaded  internally  at  the  upper  end  to  fit  the  closing 
plug  (9).  The  upper  end  of  8  and  the  lower  end  of  the  larger  portion  of 
9  are  made  concave  in  form,  and  between  them  is  placed  the  packing 
(10).  The  concave  form  of  these  two  surfaces  is  essential,  since  it  pre- 

*Amer.  Chem.  Journal,  XL,  266;  XLV,  91. 


CELLS   AND   MANOMETER   ATTACHMENTS.  21 

vents  any  outward  lateral  movement  of  the  packing  and  causes  the 
latter  to  close  up  tightly  on  the  thread  of  the  screw.  After  fixing  the 
needle  and  the  manometer  tube  (or  rather  the  tube  (5)  which  is  to  be 
fused  to  the  manometer)  in  their  places,  the  cone  (4)  is  extended  by 
means  of  the  fusible  metal  (11)  in  order  to  protect  the  lower  end  of  the 
needle.  Over  the  cone,  thus  extended,  is  slipped  the  rubber  tube  (12) 
which  is  tightly  wound  at  the  lower  and  upper  ends  (13  and  14)  with 
twisted  shoemakers'  thread.  Owing  to  the  ease  with  which  rubber 
moves  in  the  direction  of  smaller  pressure,  the  whole  space  (14)  between 
the  shoulder  of  the  brass  cone  and  the  top  of  the  cell  must  be  covered  and 
rigidly  supported  by  the  thread.  In  practice,  the  winding  of  the  upper 
end  of  the  rubber  tube  is  carried  so  far  down  that  one  or  more  turns  of 
the  thread  are  forced  into  the  tapered  neck  of  the  cell. 

A  manometer,  on  whose  calibration,  capillary  depression,  and  final 
verification  weeks  and  perhaps  months  of  labor  have  been  bestowed,  is 
too  precious  an  instrument  to  be  unnecessarily  exposed  to  danger. 
Hence  the  cones  are  not  attached  in  the  first  instance  to  the  manometers, 
but  always  to  short  pieces  of  tubing  of  the  same  kind,  which  are  afterwards 
fused  to  the  manometers  or  cut  off  from  them,  as  the  occasion  may  arise. 

At  low  and  moderate  temperatures,  the  arrangement  just  described 
renders  very  satisfactory  service,  and  between  0°  and  60°  it  is  still  in 
use.  At  higher  temperatures,  it  develops  certain  defects  which  are  so 
serious  as  to  render  its  use  quite  impracticable.  Leaks  appear,  due  to 
increasing  difference  between  the  expansion  coefficients  of  brass,  glass, 
and  the  fusible  metal;  the  alloy  attacks  the  brazing  or  solder  used  in 
attaching  the  hollow  needle  to  the  brass  piece  (Figure  9) ;  the  glass  of  the 
manometers  becoming  brittle  after  continued  use  at  high  temperatures, 
it  is  difficult  to  fuse  them  on  the  glass  tubes  which  pass  through  the 
cones;  finally,  at  high  temperatures,  the  rubber,  used  between  the  brass 
cone  and  the  neck  of  the  cell,  also  becomes  brittle  and  liable  to  crack. 

The  deterioration  of  rubber  at  moderately  elevated  temperatures — - 
apparently  due,  in  our  case,  to  a  resumption  and  continuation  of  the 
vulcanizing  process  in  the  baths — has  given  much  trouble,  but  we  have 
not  been  able  wholly  to  dispense  with  its  use.  We  are  able  to  make  tight 
joints  without  it,  but  not,  as  yet,  any  satisfactory  device  for  adjusting 
pressure  in  the  cell. 

The  remainder  of  the  manometer  attachments  which  are  here 
described  were  devised  for  use  at  the  higher  temperatures  or  with  elec- 
trolytes, though  they  render  equally  good  service  at  moderate  and  low 
temperatures,  and,  of  course,  also  with  non-electrolytes. 

In  Figure  10,  the  cone  (a),  which  closes  the  neck  of  the  cell  (A),  is 
turned  on  the  lower  end  of  the  brass  tube  (B).  At  b  there  is  a  vent  for 
the  escape  of  air  and  any  excess  of  solution.  The  usual  collar  and  nut 
for  fixing  the  manometer  in  the  cell  and  for  adjusting  the  pressure  are 
seen  at  c  and  d.  The  manometer  ((7)  is  held  tightly  in  its  place  in  the 


22 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS, 
c 


11 


FIG.  10. — Third  form  of  complete  cell. 
(A)   Porous  cell — upper  half  glazed;    (B)  brass  piece;  (C)  manometer;    (a)   conical  end  of  (B); 

(b)  vent  for  solution;  (c)  brass  collar;  (d)  brass  nut  resting  on  ledge  of  (B);  (e)  and  (e)  brass 
rings  between  which  packing  is  placed;  (/)  brass  collar  screwing  down  upon  (B)  and  com- 
pressing the  packing  between  the  rings  (e) ,  (e)  •  (g)  brass  ring. 

FIG.  11. — Fourth  form  of  complete  cell. 

(a),  (a)  Brass  or  porcelain  rings  for  compressing  the  packing  and  displacing  it  laterally;  (6)  brass 
tube  around  which  all  metallic  parts  are  assembled,  upper  end  the  same  as  in  Fig.  10; 

(c)  brass  piece  employed  in  compressing  packing  and  in  adjusting  initial  pressure;  (e)  brass 
collar;  (/)  brass  ring;  (g)  brass  nut,  threaded  internally,  which  is  employed  in  adjusting 
initial  pressure  by  moving  the  tube  (6). 


CELLS   AND    MANOMETER   ATTACHMENTS. 


23 


tube  (B)  by  packing  which  is  compressed  between  the  rings  (e,  e). 
form  of  the  rings  is  such  as  to  force  the  material 
of  the  packing  in  both  of  the  lateral  directions — on 
the  one  side  toward  the  manometer,  and  on  the 
other  toward  the  wall  of  the  brass  tube.    The  com- 
pression is  effected  by  means  of  the  hollow  nut  (/) . 
The  brass  ring  (g),  which  serves  as  a  "follower," 
is  made  of  any  required  length.     A  rubber  tube 
is  slipped  over  the  cone  (a)  and  tied  above  and  be- 
low the  neck  of  the  cell  in  exactly  the  same  manner 
as  in  the  apparatus  shown  in  Figure  9.    The  reason 
for  the  concave  form  of  the  surfaces  between  which 
the  packing  of  the  vent  (b)  is  compressed  has  al- 
ready been  explained.     The  packing  between  the 
rings  (e,  e)  usually  consists  of  alternate 
disks  of  leather  and  thin  rubber,  one 
of  the  rubber  disks  being  placed  below 
the  lower  ring,  i.  e.,  between  it  and  its 
1  'seat. ' '    The  seat  and  the  under  side  of 
the  lower  ring  are  grooved  to  prevent 
too  much  lateral  movement  on  the  part 
of  the  rubber  between  them  in  the 
direction  of  the  manometer;  otherwise 
the  greater  part  of  the  material  of  this 
lowest  disk  would  be  crowded  into  the 
cavity  below  the  ring.     All  brass  sur- 
faces which  are  exposed  to  the  liquid  con- 
tents of  the  cell  are  plated  with  nickel,  silver, 
or  gold,  according  to  the  character  of  the 
solutions  whose  pressure  is  to  be  determined. 

The  arrangement  shown  in  Figure  10  has 
two  great  advantages  over  that  presented 
in  Figure  9.     The  use  of  fusible  metal  is 
avoided,  and,  if  the  right  kind  of  packing  is 
used,  it  is  not  necessary  at  any  time  to  sepa- 
rate the  calibrated  end  of  the  manometer 
from  the  end  entering  the  cell,  since  the 
manometer  is  always  sufficiently  released 
by  unscrewing  the  nut  (/)  to  permit  of 
its  easy  withdrawal  from  the  tube  (E). 
It  also  does  away  with  the  plated  steel 
needle    (Figure   9,e),  which  may  be 
corroded  if  the  solution  contains  an 
electrolyte. 

In  the  apparatus  seen  in  Figure  11 
the  cone  (Figure  10,  a,  a,)  is  dispensed 


The 


FIG.  12. — Fifth  form  of  complete  cell;  for 
use  with  substances  which  attack  metals. 

(a)  Enlarged  end  of  manometer;  (6)  brass 
piece  fastened  to  manometer  by  litharge- 
glycerine  cement;  (c)  hollow  brass  nut 
resting  upon  (6) ;  (d)  brass  collar;  (e)  vent 
for  solution;  (/)  brass  cap  with  packing 
in  bottom. 


24  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 

with,  and  the  union  between  the  manometer  attachment  and  the  cell 
is  effected  by  means  of  the  brass  rings  (a,  a,  a,  a)  and  the  packing 
which  is  placed  between  them.  The  packing  is  compressed  in  the  ver- 
tical direction  and  made  to  expand  horizontally  against  the  cell  on  the 
one  side  and  the  brass  tube  (6,  6)  on  the  other  by  the  sliding  piece  (c,  c). 
The  collar  (e,  e)  and  the  nut  (/,  /)  do  not  differ  essentially  from  the  corre- 
sponding pieces  seen  in  Figures  9  and  10.  The  vent  and  the  arrange- 
ments for  fixing  the  manometer  are  the  same  in  Figure  11  as  in  Figure  10. 
The  adjustment  of  pressure  within  the  cell  is  effected  by  means  of  the 
nut  (g,  g).  Turned  to  the  right,  it  drives  the  tube  (6,  6),  and  with  it  the 
manometer,  into  the  cell,  increasing  the  pressure.  If  it  is  turned  to  the 
left,  the  tube  and  manometer  are  raised  and  the  pressure  diminished. 
The  principal  advantages  of  the  arrangement  seen  in  Figure  11  over  that 
shown  in  Figure  10  are  in  the  better  means  of  adjusting  the  pressure 
and  in  the  substitution  of  packing  for  rubber  tubing  in  making  the 
joint  with  the  cell. 

In  measuring  the  osmotic  pressure  of  electrolytes,  it  is  desirable  to 
avoid,  as  far  as  possible,  any  contact  of  the  solutions  with  metallic  sur- 
faces, even  though  the  same  are  protected  by  plating  with  the  more 
resistant  metals.  The  covering  is  often  imperfect  in  spots,  notwith- 
standing the  care  which  is  taken  in  the  plating.  Accordingly,  a  number 
of  schemes  have  been  devised  for  joining  the  manometer  and  the  cell,  in 
which  the  solution  comes  in  contact  only  with  glass  and  rubber. 

In  Figure  12,  the  hollow  glass  cone  (a)  serves  the  same  purpose  as  the 
brass  cones  seen  in  Figures  9  and  10.  It  is  set  in  the  brass  piece  (6) 
with  litharge-glycerine  cement.  Its  use,  in  connection  with  the  usual 
collar  (d)  and  nut  (c),  is  apparent.  The  cone,  like  those  in  Figures  9 
and  10,  is  covered  with  rubber  tubing,  which  is  wound  and  tied  at  the 
upper  and  lower  ends  with  twisted  shoemakers'  thread.  The  side  tube 
(e),  which  serves  as  a  vent  for  the  escape  of  surplus  solution,  is  embedded 
with  cement  in  a  brass  tube,  which  is  threaded  externally  to  receive  the 
cap  (f) .  The  packing  in  the  bottom  of  the  cap  closes  the  vent.  It  will 
be  seen  that  the  means  of  adjusting  pressure  within  the  cell  is  the  same 
in  all  three  of  the  instruments  represented  in  Figures  9,  10,  and  12. 

Another  form  of  glass  cone  which  has  rendered  good  service  is  seen 
in  Figure  13.  The  cone,  which  is  made  of  a  solid  piece  of  glass,  is  bored 
excentrically  for  the  manometer  (a)  and  the  vent  (6).  The  vent  is 
closed  at  the  lower  end  by  the  rubber  disk  (e),  which  is  attached  to  and 
controlled  by  a  platinum  rod  running  through  the  hole  in  the  stopper. 
The  upper  end  of  the  rod  is  threaded  and  provided  with  a  nut,  as  seen 
in  the  figure.  To  prevent  the  solution  which  escapes  through  the  vent 
from  coming  into  direct  contact  with  the  cement,  all  exposed  parts  of  the 
latter  are  painted  with  a  solution  of  rubber.  The  solid  glass  cone  has 
some  advantages  over  the  hollow  one,  as  will  appear  when  the  manipu- 
lation connected  with  filling  and  closing  the  cells  is  explained. 


CELLS   AND   MANOMETER   ATTACHMENTS. 


25 


15 


FIG.  13. — Solid  glass  stopper  for  use  with  substances  which  attack  metals, 
(a)  Manometer  tube;  (6)  vent  for  solution,  closed  by  valve  at  lower  end  of  stopper. 

FIG.  14. — Glass  manometer  attachment  for  cells  with  straight  necks. 

(a)  Manometer  with  straight  tube  fused  to  lower  end;  (6)  space  between  manometer  and  glass 
tube;  (c)  brass  ring;  (d)  and  (e)  porcelain  rings  for  compressing  packing;  (/)  brass  collar; 
(0)i  C')i  (*')>  an^  0').  brass  pieces  with  which  to  close  the  cell,  and  also  to  adjust  initial  pressure; 
(k)  vent  for  solution. 

FIG.  15. — Glass  manometer  attachment  for  cells  with  straight  neck. 

Like  that  shown  in  Fig.  14,  except  that  the  glass  tube  is  left  open  at  the  top,  and  then  closed 
with  a  brass  cap  and  litharge-glycerine  cement. 


26       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

Still  another  glass  device  for  attaching  the  manometer  to  the  cell  is 
seen  in  Figure  14.  It  was  designed  for  use  with  the  form  of  cell  which 
is  seen  in  Figure  11.  The  manometer  (a)  passes  entirely  through  the 
closed  tube  (6),  whose  outside  diameter  is  only  a  little  less  than  the 
interior  diameter  of  the  cell.  The  means  for  compressing  the  packing 
(c,  d,  e,  f,  and  g)  and  fixing  the  large  tube  (6)  in  the  cell  do  not  differ 
essentially  from  the  analogous  parts  seen  in  Figure  11,  except  that  the 
lower  ring  (e)  is  made  of  porcelain,  in  order  that  the  solution  in  the 
cell  may  nowhere  come  in  contact  with  metal.  The  adjustment  of 
pressure  within  the  cell  is  effected  by  means  of  the  brass  pieces  (h, 
which  is  fixed  in  its  place  with  cement) ,  (i),  and  ( j~) .  The  vent  (k)  is  not 
absolutely  necessary,  though  it  is  sometimes  a  convenience.  Instead 
of  opening  the  vent  when  it  is  desired  to  lessen  the  pressure,  the  nut 
(j)  may  be  turned  slightly  to  the  left. 

The  device  shown  in  Figure  15  is  a  substitute  for  that  seen  in  Figure 
14.  The  two  differ  only  in  that  the  large  tube  (6)  is  closed  at  both 
ends  in  Figure  14,  while  it  is  open  at  the  top  in  Figure  15.  The  latter 
is  easier  to  make,  and  is  in  no  way  inferior  to  the  closed  form. 


CHAPTER  II. 
THE  MANOMETERS. 

The  possibility  of  correctly  determining  osmotic  pressure  depends 
upon  four  fundamental  conditions,  no  one  of  which  can  be  said  to 
exceed  another  in  importance.  They  are  (1)  a  suitable  cell,  i.  e.,  a  cell 
which  is  able  to  support  the  membrane  under  high  pressure  and  in 
which  the  membrane  is  always  deposited  upon  the  interior  surface  of 
the  porous  wall;  (2)  a  truly  semi-permeable  membrane,  i.  e.,  a  mem- 
brane which  does  not  leak  the  solute;  (3)  a  perfectly  automatic  and 
exact  regulation  of  temperature;  and  (4)  an  accurate  calibration  of  the 
manometers.  If  any  one  of  these  conditions  is  unfulfilled,  all  efforts 
to  measure  the  force  must  lead  to  erroneous  results,  which  are  not  only 
futile  but  positively  mischievous — mischievous  because  they  furnish 
the  opportunity  for  an  indulgence  of  the  propensity  of  the  over-hasty 
and  unwary  to  erect  elaborate  speculative  structures  upon  foundations 
of  what  may  be  justly  called  tainted  facts. 

The  manometers  which  are  used  for  the  measurement  of  osmotic 
pressure  have  an  external  diameter  of  about  6  millimeters.  The  length 
of  the  calibrated  portion  varies  from  400  to  500  millimeters.  The 
diameter  of  the  bore  ranges  from  0.45  to  0.72  millimeter. 

The  reasons  for  using  tubes  of  very  small  bore  are : 

1.  It  is  necessary  to  fill  the  upper  ends  of  the  manometers  with  short 
columns  of  mercury,  because  in  closing  the  instruments,  after  the  intro- 
duction of  the  gas,  the  caliber  of  the  tubes  in  that  region  is  affected  to  an 
unknown  extent.    If  the  internal  diameter  is  large,  e.  g.,  1 .0  millimeter  or 
more,  the  mercury  is  often  dislodged  by  the  severe  tapping  to  which  the 
manometers  are  subjected  at  certain  times. 

2.  The  compression  of  the  small  volume  of  gas  which  they  contain 
involves  but  little  dilution  of  the  cell  contents. 

3.  Relatively  small  volumes  of  mercury  are  required  by  manometers 
of  small  bore.     The  importance  of  this  fact  will  be  better  understood 
when  the  subject  of  "thermometer  effects"  is  discussed. 

The  disadvantages  of  using  manometers  of  small  bore  are: 

1.  It  is  more  difficult  to  deal  satisfactorily  with  the  meniscus  in  a 
narrow  tube. 

2.  The  capillary  depression  is  large  in  small  tubes  and  it  varies 
greatly  with  slight  irregularities  of  bore. 

3.  The  movements  of  the  mercury  in  narrow  tubes  are  strongly 
influenced  by  the  presence  of  minute  quantities  of  impurities,  whether 
the  same  are  dissolved  in  the  metal  itself  or  are  attached  to  the  surface 

of  the  glass. 

27 


28  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 

PURIFICATION  OF  THE  MERCURY. 

The  material  which  ordinarily  passes  for  pure  mercury  in  the  labora- 
tory is  by  no  means  suitable  for  manometric  work,  and  to  obtain  it 
in  adequately  pure  condition  for  this  purpose  requires  unusually 
thorough  treatment.  The  mercury  which  is  used  in  our  manometers— 
and  also  that  which  is  now  used  in  the  bath  thermostats — is  cleansed 
in  the  following  manner: 

1.  The  commercial  material  is  first  filtered  through  paper  filled  with 
pin  holes  to  free  it  from  dirt.     It  is  then  heated  for  four  hours  to  the 
boiling-point  in  a  glass  retort,  to  the  neck  of  which  a  long  glass  tube 
has  been  fused  for  the  condensation  and  return  of  the  vapors;  and 
during  this  time  a  current  of  air  is  forced  through  the  boiling  metal. 
On  cooling,  it  is  again  filtered  to  remove  the  scum  of  oxides  which 
usually  forms  in  considerable  quantity. 

2.  It  is  distilled  in  a  vacuum. 

3.  The  distillate  is  washed  by  the  method  of  Lothar  Meyer,  but 
with  water  containing  2  per  cent  of  nitric  acid  and  2  per  cent  of  mer- 
curous  nitrate  instead  of  ferric  chloride.    The  apparatus  in  which  the 
washing  is  done  consists  of  a  wide  tube  two  meters  in  length,  to  the 
lower  end  of  which  has  been  fused  a  quite  narrow  tube  of  the  usual 
double  U  form,  the  proportions  of  the  descending  and  ascending  limbs 
being  so  selected  that  the  mercury  which  supports  the  cleansing  liquid 
shall  lie  wholly  within  the  smaller  tube.     To  admit  the  mercury  at 
the  top  and  to  regulate  its  flow,  a  separating  funnel  is  employed.     The 
lower  end  of  the  funnel,  instead  of  being  drawn  out  to  a  fine  point,  as 
in  the  apparatus  of  Meyer,  is  widened  out  into  the  form  of  an  inverted 
funnel,  according  to  the  suggestion  of  Hillebrand,  and  over  this  are  tied 
two  or  three  thicknesses  of  the  finest  silk  bolting-cloth.     The  material 
to  be  purified  is  thus  made  to  enter  the  cleansing  liquid  in  hundreds  and 
perhaps  thousands  of  excessively  fine  streams.     It  is  passed  1,000  times 
through  the  solution  of  nitric  acid  and  mercurous  nitrate,  and  is  then 
thoroughly  washed  with  water  and  dried. 

4.  After  treating  the  mercury  as  described  under  1,  2,  and  3,  it  is 
again  distilled  in  a  vacuum,  but  not  in  the  still  (2)  which  is  used  for 
the  first  distillation. 

The  mercury  which  has  thus  been  cleansed  retains  its  brilliant  luster 
in  the  air,  and  its  movements  in  narrow  tubes  are  highly  satisfactory. 
We  have  also  prepared  mercury  from  the  purest  oxide  which  we  could 
make,  but  have  not  found  it  superior  in  any  way  to  the  product 
obtained  by  the  means  desribed  above. 

CALIBRATION  OF  THE  MANOMETERS. 

The  tubes  which  are  used  in  making  the  manometers  are  the  most 
nearly  perfect  for  the  purpose  which  it  is  practicable  to  obtain.  The 
essential  requirements  are  that  any  tube  shall  be  of  yery  nearly  uni- 


THE   MANOMETERS.  29 

form  bore  throughout,  and  that  the  form  of  the  bore  in  every  part  shall 
be  circular.  Very  few,  if  any,  tubes  conform  perfectly  to  both  require- 
ments. The  material  from  which  selections  are  to  be  made  is  imported 
in  lots  of  several  kilograms  each,  and  the  purveyors  are  urged  to  spare 
neither  pains  nor  expense  in  procuring  tubes  of  the  highest  possible 
excellence.  In  each  lot  of  selected  material  thus  obtained,  there  are 
usually  found — though  not  always — a  few  tubes  which  answer  all  rea- 
sonable requirements. 

The  first  step  in  making  a  manometer  is  to  etch  upon  the  tube  two 
fine  lines  extending  completely  around  the  instrument.  These  are 
usually  referred  to  as  the  "upper  scratch"  and  "lower  scratch,"  one 
being  near  the  upper  and  the  other  near  the  lower  limit  of  the  calibrated 
portion  of  the  manometer.  These  lines  are  made  no  coarser  than  is 
absolutely  necessary  in  order  that  they  may  be  distinctly  seen  through 
the  telescope,  since  in  small  tubes  a  meniscus  behind  any  line,  however 
fine,  is  apt  to  give  the  observer  trouble.  No  other  graduation  appears 
upon  the  manometers.  All  readings  on  the  instruments  are  referred  to 
one  or  the  other  of  the  two  "scratches."  That  is,  a  reading  consists 
always  in  determining  the  distance  between  the  meniscus  of  a  mercury 
column  and  either  one  of  the  lines  in  question.  Since  the  distance 
between  them  is  accurately  known,  readings  referred  to  one  line  can 
readily  be  transferred  to  the  other.  The  distance  between  the  lines 
depends  upon  the  length  which  the  manometer  is  to  have — ultimately, 
of  course,  upon  the  height  of  the  available  space  in  the  baths.  Above 
the  upper  and  below  the  lower  scratch,  a  considerable  length  of  tube 
is  left  to  provide  for  subsequent  operations. 

Two  methods  of  calibration  have  been  employed,  both  of  which  will 
be  briefly  explained,  though  the  earlier  one  is  not  now  much  in  use 
except  for  preliminary  explorations  of  the  tubes. 

FIRST  METHOD. 

Figure  16  represents  the  instrument  which  is  employed  to  move  and 
adjust  the  calibrating  thread.  A  steel  screw  (a),  with  a  long  lever,  is 
threaded  through  a  cap  of  hard  rubber  (&),  in  which  the  glass  tube  (d) — 
enlarged  at  c — is  set  with  litharge-glycerine  cement.  In  order  to  make  a 
mercury-tight  joint,  the  upper  end  of  the  steel  screw  is  slightly  lubri- 
cated, and  around  the  portion  which  extends  into  the  glass  tube  some  of 
the  cement  is  allowed  to  solidify.  The  rubber  stopper  (/),  carrying  the 
manometer  (</),  is  inserted  (with  the  aid  of  the  "fang,"  Figure  8)  in  the 
glass  tube  (d),  which  is  sharply  contracted  at  the  upper  end  (e). 

The  manometer  is  drawn  out  at  the  upper  end  to  a  fine  tube  which  is 
bent  into  the  form  of  an  inverted  U.  With  the  apparatus — including 
the  manometer — nearly  full  of  mercury,  the  screw  is  turned  to  the  right 
until  the  column  enters  and  reaches  the  highest  part  of  the  inverted  U . 


30 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


The  outlet  is  then  immersed  in  a  globule  of  pure  mercury  and  the 
screw  is  reversed.  This  manipulation  brings  a  short  calibrating  thread 
(h)  into  the  manometer,  which  is  separated  from  the  main  body  of  the 
mercury  by  a  cushion  of  air.  The  thread  can 
now  be  made  to  take  any  desired  position  in  the 
tube  by  simply  turning  the  screw  and  simulta- 
neously tapping  the  manometer.  The  electric 
"hammer"  employed  for  the  latter  purpose  will 
be  described  in  connection  with  the  process  for 
the  determination  of  capillary  depression. 

The  process  of  calibration  consists  in  bringing 
the  lower  end  of  the  thread  to  the  point  at  which 
it  is  desired  to  begin,  and  then  setting  it  exactly 
end  to  end  up  the  tube,  determining  each  time  the 
length  of  the  detached  column.  When  the  cali- 
bration has  been  carried  as  far  up  as  is  desired, 
the  thread  is  run  out  and  weighed.  Subsequently 
a  long  thread  of  mercury,  one  filling  nearly  the 
whole  length  of  the  calibrated  portion  of  the  tube, 
is  drawn  in  and  measured,  and  then  run  out  and 
weighed.  It  is  evident  that  the  weight  of  the  short 
thread,  multiplied  by  the  number  of  settings,  will 

11         AI_        J.T!    -L     rj.il     i  J.T-         J^iv         j-u  FIG.  16. — First  arrangement 

be  less  than  that  ot  the  long  thread  filling  the  same  f or  calibrating  manometers, 
length  of  tube,  by  the  weight  of  the  mercury  re-  (a)  screw  for  setting  caii- 
quired  to  fill  the  double  meniscus  spaces.  By  brating  thread;  (6)  hard- 

f  ,-,  .          •,    ,.  ,,  ,•         f        ,i  rubber  cup;   (c)   enlarge- 

means  of  this  relation,  the  correction  for  the  vol- 
ume of  the  double  meniscus  is  readily  calculated. 


SECOND  METHOD. 


ment  in  glass  tube  (i); 
(d)  litharge-glycerine  ce- 
ment; (e)  contracted  end 
of  glass  tube;  (/)  rubber 
stopper;  (g)  manometer; 
(h)  calibrating  thread  sep- 
arated from  main  column 
of  mercury  by  air ;  (i)  glass 
tube  filled  with  mercury. 


The  later  procedure  differs  from  the  earlier  one 
in  manner  rather  than  in  principle.  After  etching 
upon  the  glass  the  two  lines  previously  men- 
tioned, a  small  bulb  is  blown  near  each  end  of  the  tube  outside  the 
portion  to  be  calibrated.  These  serve  to  catch  and  preserve  the  cali- 
brating thread  in  case  of  accident.  For  calibration,  the  tube  is  placed 
in  the  horizontal  position,  over  a  ruled  mirror,  on  the  dividing  engine, 
the  screw  of  which  has  been  carefully  compared  with  the  graduated 
meter  scales  employed  in  the  measurement  of  osmotic  pressure. 

The  device  employed  for  shifting  the  thread  from  one  position  to 
another  is  shown  in  Figure  17.  A  is  the  manometer  with  its  two  bulbs 
(a,  a).  The  two  lines  of  reference  previously  referred  to  as  the 
"scratches"  are  seen  at  6, 6.  The  shifting  arrangement  (B)  for  the  cali- 
brating thread  (c)  consists  of  a  steel  ball  (d),  a  large  bicycle  ball,  which 
is  located  in  the  center  of  a  rubber  tube  (e).  A  and  B  are  connected 
through  the  glass  tubes  (/,/)  and  the  rubber  tubes  (g,  g}. 


THE   MANOMETERS.  31 

If  it  is  desired  to  move  the  thread  to  the  right,  the  rubber  tube  (e), 
to  the  left  of  the  ball  (d),  is  compressed  between  the  thumb  and  fore- 
finger of  the  left  hand  until  the  meniscus  has  taken  the  right  position 
under  the  miscroscope,  when,  without  releasing  the  tube,  the  rubber  over 
the  ball  (d)  is  pinched  between  the  thumb  and  forefinger  of  the  right 
hand  until  a  passage  for  air  is  opened.  The  portion  of  the  rubber  tube 
which  is  held  in  the  left  hand  may  then  be  released,  since  any  difference 
in  atmospheric  pressure  at  the  two  ends  of  the  thread  is  quickly  equal- 
ized through  the  passage  which  has  been  opened  over  the  ball  (d),  and 
without  disturbing  the  thread.  If  the  thread  is  to  be  moved  to  the 
left,  the  rubber  tube  to  the  right  of  d  is  compressed  between  the  fingers 
of  the  right  hand,  and  the  passage  for  air  over  the  ball  is  made  with  the 
left  hand.  After  a  little  experience,  the  exact  adjustment  of  the  cali- 
brating thread  becomes  easy  and  nearly  automatic. 


_LEJ 


FIG.  17. — Second  arrangement  for  calibrating  manometers. 

(A)  Tube  to  be  calibrated  (bore  from  0.45  to  0.65  millimeter);  (B)  rubber  tube;  (a,  a)  bulbs 
blown  on  each  end  of  tube  to  prevent  loss  of  calibrating  thread;  (b,  6)  lines  of  reference 
etched  on  tube;  (e)  calibrating  thread;  (d)  steel  ball  for  setting  calibrating  thread;  (e)  rubber 
tubing;  (/,  f)  glass  tubes;  (g,  g)  rubber  tubes. 

The  calibration  is  commenced  somewhat  below  the  lower  scratch — 
the  etched  line  to  the  left — and  consists,  as  when  the  tube  is  calibrated 
in  the  vertical  position,  in  setting  the  thread  exactly  end  to  end  and 
determining  its  length  until  the  thread  has  passed  the  upper  scratch. 
It  is  then  run  out  of  the  tube  and  weighed.  Afterwards  the  whole 
of  the  calibrated  portion  of  the  tube  is  filled  with  mercury,  which  is 
also  run  out  and  weighed. 

From  the  length  and  weight  of  the  long  thread,  the  mean  diameter 
of  the  bore  is  calculated;  and  from  the  observations  on  the  length  of 
the  short  thread  in  the  different  parts  of  the  tube,  a  mean  calibration 
unit  is  derived,  and  a  curve  of  corrections  constructed,  exactly  as  in 
the  calibration  of  a  eudiometer.  Finally,  a  mean  value  for  the  double 
meniscus  is  obtained  from  the  length  and  weight  relations  of  the  long 


32 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


and  short  threads.  If  we  multiply  the  weight  of  the  short  thread  by 
the  number  of  times  its  length  is  contained  in  that  of  the  long  thread, 
i.  e.,  by  the  number  of  times  it  was  set  end  to  end,  and  subtract  the 


FIG.  18. — Simplest  form  of  manometer. 

(1)  and  (2)  bulbs  with  traps  in  the  bottom  to  prevent  liquids  from  working  their  way  into  the 
calibrated  portion  of  the  instrument;  (3)  nitrogen  reservoir  to  prevent  loss  of  gas  under 
diminished  pressure;  (4)  mercury  filling  the  portion  of  tube  whose  caliber  may  have  been 
altered  in  closing  the  instrument. 

FIG.  19. — Manometer  for  high  pressure. 

Differs  from  that  in  Fig.  18  only  in  having  a  nitrogen  reservoir  within  the  calibrated  portion  of 

the  instrument. 

product  from  the  weight  of  the  long  thread,  the  difference  is  the  weight 
of  the  mercury  which  would  be  required  to  fill  all  the  meniscus  spaces 
which  were  left  vacant  in  setting  the  short  thread  end  to  end  along 
the  tube.  Converting  this  difference  in  weight  into  volume,  and  divid- 


THE   MANOMETERS.  33 

ing  by  the  number  of  settings  less  one,  we  obtain  a  mean  correction 
for  a  double  meniscus,  which  is  the  meniscus  correction  to  be  applied 
in  all  measurements  of  pressure,  since  the  nitrogen  in  the  manometers 
is  always  included  between  two  mercury  columns. 

The  method  which  is  explained  above  suffices  for  the  simple  form  of 
manometer  seen  in  Figure  18,  but  some  modifications  are  necessary 
when  a  manometer  of  the  form  seen  in  Figure  19  is  to  be  calibrated. 
The  peculiarity  of  the  latter  instrument  is  the  large  reservoir  for  gas 
which  lies  between  the  two  lines  of  reference  and  within  the  calibrated 
area.  The  narrower  portions — below,  from  some  point  under  the  lower 
scratch  to  the  bottom  of  the  enlargement,  and  above,  from  the 
top  of  the  wide  part  to  the  end  of  the  tube — are  calibrated  in  the  man- 
ner already  described.  The  meniscus  correction  also  is  derived  from 
the  weight  and  length  relations  of  short  and  long  threads.  So  far  the 
procedure  is  without  change.  It  remains,  however,  to  ascertain  the 
capacity  of  the  wider  part  as  a  whole,  and  eventually  in  terms  of 
the  calibration  unit.  To  do  this,  the  wider  part  is  slightly  more  than 
filled  with  mercury,  so  that  both  the  upper  and  lower  meniscus  are  well 
within  calibrated  portions  of  the  narrow  ends.  From  the  weight  of 
this  mercury — with  proper  correction  for  overlapping  in  the  narrower 
calibrated  parts — the  total  capacity  of  the  wider  part  of  the  tube  is 
calculated.  Two  verifications  of  the  correctness  of  the  previous  work 
are  now  undertaken.  It  will  be  noticed,  on  referring  to  Figure  19, 
that  the  upper  line  of  reference  is  not  very  far  above  the  upper  end 
of  the  wider  portion  of  the  manometer.  The  first  step  in  the  verifica- 
tion is  to  fill  the  space  between  the  two  scratches  with  mercury — the 
upper  meniscus  may  lie  somewhat  above  the  upper  scratch.  The 
volume  of  this  mercury  should,  of  course,  be  equal  to  the  sum  of  the 
previously  found  capacities  of  all  of  the  parts  which  were  filled  by  it. 
The  final  step  in  the  verification  is  to  apply  the  same  test  to  the  whole 
tube  by  filling  it  with  mercury  from  the  lower  scratch  to  the  upper 
limit  of  the  calibration. 

THE  MENISCUS. 

In  narrow  tubes,  owing  to  the  small  volume  of  the  gas  which  they 
contain,  the  meniscus  correction  is  of  considerable  importance,  since 
it  may  amount — especially  at  high  pressures — to  an  appreciable  frac- 
tion of  the  volume  of  the  gas. 

The  significance  of  the  meniscus  correction,  when  translated  into 
pressure,  increases  with  increasing  concentration  of  the  solutions  with 
a  rapidity  which  might  well  astonish  one  who  has  not  clearly  in  mind 
the  fact  that,  though  in  the  first  instance  it  is  simply  a  space  of  fixed 
volume,  its  importance  depends,  not  only  on  the  pressure  upon  the 
gas  which  fills  it,  but  also  upon  the  volume  of  all  the  gas  in  the  manom- 
eter. The  effect  of  this  relation  in  practice  is  illustrated  by  means  of 


34 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


the  following  tabulation  of  data  taken  from  the  record  of  a  single 
manometer  (No.  9).  The  meniscus  correction  (double)  in  this  instru- 
ment is  0.17  calibration  unit,  and  the  volume  of  the  nitrogen  under 
standard  conditions  of  temperature  and  pressure  is  454.14  calibration 
units.  The  temperature  in  all  cases  is  25°.  Column  I  in  Table  1  gives 
the  weight-normal  concentration  of  the  solutions;  II  gives  the  observed 
pressure  in  atmospheres;  III  shows  the  volumes  of  the  compressed 
nitrogen  reduced  to  standard  temperature;  IV,  the  corrections  in 
fractions  of  an  atmosphere  for  the  double  meniscus;  V,  the  relative 
osmotic  pressures,  the  pressure  of  the  0.1  normal  solution  being  taken 
as  unity;  and  VI,  the  relative  corrections  for  meniscus,  the  correction 
for  the  0.1  normal  solution  serving  as  the  unit. 

TABLE  1. 


I. 

II. 

III. 

IV. 

V. 

VI. 

Concen- 
tration. 

Osmotic 
pressure, 
atmospheres. 

Vol.  N2 
cal.  units. 

Meniscus 
correction, 
atmosphere. 

Relative 
osmotic 
pressure. 

Relative 
meniscus 
correction. 

0.1 

2.635 

141.15 

0.00317 

1.0000 

1.000 

0.2 

5.139 

80.69 

0.01083 

1.9503 

3.4164 

0.3 

7.738 

55.59 

0.02366 

2.9366 

7.4637 

0.4 

10.295 

42.41 

0.04126 

3.9070 

13.0158 

0.5 

12.947 

34.01 

0.06972 

4.9135 

21.9937 

0.6 

15.620 

28.37 

0.09360 

5.9275 

29.5268 

0.7 

18.436 

24.11 

0.12999 

6.9928 

41.0063 

0.8 

21.258 

20.97 

0.17233 

8.1055 

54.3628 

0.9 

24.126 

18.53 

0.22133 

9.1558 

69.8202 

1.0 

27.076 

16.54 

0.27834 

10.2755 

87.8044 

Particular  attention  is  called  to  columns  V  and  VI,  where  it  will  be 
seen  that,  while  the  osmotic  pressure  increased  a  little  over  ten-fold, 
the  value  of  the  meniscus  correction  increased  nearly  88-fold.  Ex- 
pressed in  heights  of  a  mercury  column,  the  correction  for  meniscus 
in  the  case  cited  in  the  table  increases  from  a  value  of  2.4  millimeters 
to  one  of  211.5  millimeters. 

The  method  of  obtaining  the  meniscus  correction  which  is  given 
above  is  believed  to  be  entirely  correct  in  principle.  Nevertheless  it 
has  been  found,  in  applying  it,  that  the  calculated  volume  of  the 
meniscus  is  always  less  than  it  would  have  been  if  the  form  of  the 
meniscus  were  truly  spherical,  as  it  is  generally  assumed  to  be.  The 
experimental  correction  is  usually  just  about  three-fourths  that  calcu- 
lated from  the  supposed  spherical  form  of  the  meniscus.  The  difference 
may  be  due  to  unavoidable  errors  in  reading  the  length  of  the  short 
calibrating  threads.  If  these  are  always  read  "too  short,"  the  obvious 
result  would  be  a  too  small  correction  for  the  meniscus.  However, 
the  error,  if  error  it  is,  is  not  of  a  cumulative  character.  Moreover,  if, 
in  calibration,  one  reads  habitually  "too  short,"  he  will  repeat  the 
offense  in  reading  pressures.  For  these  reasons,  it  is  believed  to  be 


THE   MANOMETERS.  35 

safer  to  employ  the  experimental  correction  rather  than  that  calculated 
from  the  known  diameter  of  the  tube  and  the  supposed  spherical  form 
of  the  meniscus. 

One  great  advantage  of  the  practice  of  deriving  the  meniscus  correc- 
tion from  the  calibration  data  is  the  excellent  means  which  it  affords 
of  detecting  faulty  calibration.  It  is  known  that  the  best  work  in 
calibration  leads  uniformly  to  an  approximately  fixed  value  for  the  men- 
iscus, hence  it  is  to  be  inferred,  when  another  value  is  obtained,  that  the 
calibration  which  gave  it  is  erroneous. 

The  inverse  relation  of  the  importance  of  the  meniscus  correction  to 
the  volume  of  the  gas  which  is  measured  makes  it  desirable  to  increase 
the  quantity  of  nitrogen  in  the  manometers  as  far  as  may  be  done  with- 
out creating  other  difficulties  of  a  serious  nature.  This  has  been  accom- 
plished by  the  form  of  manometer  seen  in  Figures  19,  20,  etc.,  in  which 
the  volume  of  nitrogen  is  relatively  very  large.  In  the  manometers  of 
this  kind  which  are  in  actual  use,  a  length  of  1  millimeter  in  the  wider 
part  is  about  equal  in  capacity  to  a  length  of  16  millimeters  hi  the  nar- 
rower portion  of  the  tube.  The  column  of  mercury  which  occupies  the 
closed  end  of  the  manometer,  being  in  the  narrow  portion  of  the  ma- 
nometer, is  not  easily  dislodged  by  tapping.  In  this  respect,  the  instru- 
ment seen  in  Figure  19,  etc.,  is  not  inferior  to  the  earlier  form  seen  in 
Figure  18.  During  a  measurement  of  pressure,  the  whole  of  the  nitrogen 
is  compressed  into  the  upper  and  narrower  portion  of  the  tube,  hence 
the  column  of  the  gas  is  much  longer  under  any  given  pressure  in  the 
latter  than  in  the  former  instrument,  and  the  errors  due  to  faulty  deter- 
minations of  the  value  of  the  meniscus  and  of  the  amount  of  capillary 
depression  are  correspondingly  less  serious  in  their  effects  upon  the 
accuracy  of  the  measurement. 

Manometers  like  that  shown  in  Figure  19  are  designed  more  espe- 
cially for  the  measurement  of  the  pressure  of  concentrated  solutions 
where  errors  of  meniscus  tell  heavily  on  the  results,  unless  large  vol- 
umes of  gas  are  used.  In  the  case  of  dilute  solutions,  large  gas  volumes 
are  obviously  less  necessary  as  a  means  of  minimizing  such  errors. 

The  length  of  the  wider  portion  of  the  second  form  of  manometer  is 
varied  according  to  the  range  of  pressure  which  it  is  desired  to  measure 
with  the  instrument;  e.  g.,  if  the  pressures  in  question  lie  between  4  and 
6  atmospheres,  the  wide  and  narrow  portions  are  so  related  that  the 
mercury  meniscus  will  appear  in  the  latter  at  some  pressure  slightly 
below  4  atmospheres.  In  instruments  designed  for  use  with  normal 
solutions,  on  the  other  hand,  the  nitrogen  is  not  all  compressed  into  the 
narrower  portion  of  the  tube  until  a  pressure  of  more  than  20  atmos- 
heres  has  been  reached. 

o  considerable  dilution  of  the  solution  results  from  the  larger  vol- 
ume \>f  gas  in  such  manometers,  because,  at  the  time  of  closing  the  cell, 
a  mechanical  pressure — the  so-called  initial  pressure — is  brought  to  bear 


36 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


on  the  contents,  which  is  nearly  equal  to  the  osmotic  pressure.  Hence 
the  subsequent  diminution  in  the  volume  of  the  gas  is  small.  The  only 
disadvantage  experienced  in  the  later  form  of  manometer  is  due  to  the 
larger  volumes  of  mercury  which  must  be  stored  up  in  them.  The 
"thermometer  effects,"  resulting  from  slight  fluctuations  in  the  temper- 
ature of  the  baths,  are  therefore  more  pronounced  in  them  than  in  the 
other  form  of  instrument. 


FIG.  20. — Manometer  with  glass  cone  for 
cells  with  taper  necks  (see  Fig.  12). 

(1)  Reservoir  with  trap;  (2)  reservoir  for 
expansion  of  nitrogen  under  diminished 
pressure;  (3)  vent  for  solutions. 


THE   MANOMETERS.  37 


THE  UNCALIBRATED  PORTIONS  OF  THE  MANOMETERS. 

It  will  be  noticed  (Figures  18,  19,  20,  and  21)  that  the  uncalibrated 
portion  of  all  manometers  is  provided  with  two  or  three  bulbs,  or  their 
equivalents  in  the  form  of  inserted  short  pieces  of  tubing  of  larger 
diameter.  The  bulb  nearest  the  calibrated  end  (Figures  18  and  19,  3; 
20  and  21,  2)  serves  as  a  reservoir  in  which  the  nitrogen,  when  under 
diminished  pressure,  may  expand  without  danger  of  escaping  from  the 
instrument.  Its  capacity  is  regulated  by  the  volume  of  the  gas  to  be 
accommodated,  i.  e.,  by  its  original  or  usual  volume,  and  the  maximum 
probable  amount  of  diminished  pressure  to  which  it  will  ever  be 
subjected.  The  bulbs  nearest  the  cell  (Figures  18  and  19,  i  and  2;  20 
and  21,  i)  serve  as  reservoirs  for  the  mercury  which  is  to  be  driven 
forward  in  compressing  the  nitrogen,  and  their  total  capacity  is,  there- 
fore, to  be  regulated  by  the  volume  of  the  gas  under  ordinary  conditions 
and  the  maximum  pressures  to  be  measured. 

For  reasons  which  will  appear  later,  none  of  the  bulbs  should  be 
made  unnecessarily  large.  The  requirements  of  the  situation  may  be 
reduced  to  the  simple  rule  that  some  mercury  must  be  left  in  the  bulb 
nearest  the  manometer  proper  under  the  lowest  pressure,  and  some  in 
the  bulb  nearest  the  cell  under  the  highest  pressure.  It  will  be  noticed 
that  bulbs  1  and  2  in  Figures  18  and  19,  and  their  equivalents  (1  in 
Figures  20  and  21)  in  other  manometers,  are  provided  with  traps.  By 
means  of  these,  the  mercury  is  made  to  enter  the  narrow  tubes  below  at 
points  somewhat  above  the  bottom  of  the  bulbs.  The  purpose  of  the 
arrangement  will  be  understood  from  the  following  explanation :  When 
the  solution  in  the  cell  is  under  pressure,  it  drives  the  mercury  before  it 
and  enters  to  some  extent  the  upper  end  of  the  nearest  bulb.  When  the 
pressure  is  afterwards  removed,  and  the  mercury  which  had  been 
expelled  returns,  it  is  apt  to  entangle  minute  drops  of  the  solution  be- 
tween itself  and  the  wall  of  the  bulb.  Occasionally,  during  the  subse- 
quent movements  of  the  mercury  in  the  tube,  one  or  more  of  these 
drops  will  persistently  work  its  way  forward  toward  the  calibrated  end 
of  the  manometer,  making  it  necessary,  sooner  or  later,  to  open,  cleanse, 
and  refill  the  instrument.  The  "traps"  are  an  effectual  prevention  of 
such  calamities.  Before  their  introduction,  it  was  frequently  necessary 
to  inspect  the  manometers  for  the  presence  of  these  migrating  particles 
of  liquid,  and  it  happened  at  times  that,  notwithstanding  the  greatest 
vigilance,  they  escaped  detection  until  it  was  discovered  that  the  ma- 
nometers were  no  longer  measuring  correctly.  Straight  tubes,  because 
of  their  greater  strength  (Figures  20  and  21,  i  and  2),  are  used  for 
mercury  reservoirs  instead  of  bulbs  (Figures  18  and  19,  i,  2,  and  3) 
when  high  pressures  are  to  be  measured. 

The  manometers  shown  in  Figures  18  and  19  have  no  vents.  They 
are  suitable  for  use  in  the  arrangements  seen  in  Figures  9, 10,  and  11, 


38 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


in  which  the  vents  are  provided  for  in  the  metallic  parts  of  the  appa- 
ratus. When  all  contact  of  the  solutions  with  metals  is  to  be  avoided, 
as  in  the  case  of  electrolytes,  the  vent  is  of  glass  and  is  made  a  part  of 


FIG.  21. 

Manometer  with  glass  connec- 
tion for  cells  with  straight 
necks  (see  Fig.  14). 


the  manometer,  as  in  Figures  20  and  21,  3.  It  has  been  given  a  variety 
of  positions  on  the  manometer  (see  Figures  12,  13,  14,  and  15),  but  on 
the  whole  that  seen  in  Figures  20  and  21  is  preferred.  x 


THE    MANOMETERS. 


39 


CAPILLARY  DEPRESSION. 

Before  joining  the  calibrated  to  the  uncalibrated  portion  of  the  ma- 
nometer, the  former  must  be  subjected  to  a  thoroughgoing  investigation 
of  its  capillary  depression.  The  mean  diameter  of  the  bore  of  the 
whole  tube  is  known,  that  having  been  calculated  from  the  length 
and  weight  of  the  long  thread  of  mercury  which  is  used  in  the  calibra- 
tion ;  also  the  mean  diameters  of  a  considerable  number  of  short  spaces, 
these  having  been  calculated  in  the  same  manner  from  the  weight  of 
the  short  thread  and  its  length  in  different  parts  of  the  tube.  But, 
though  such  data  are  useful  as  a  means  of  judging  the  excellence  of  the 
tube  for  manometric  purposes,  they  can  not  be  relied  upon  for  the 
derivation  of  the  capillary  depression. 

The  mean  capillary  depression  of  the  mercury  in  the  manometer  of 
smallest  bore  amounts  to  18  millimeters,  i.  e.,  to  more  than  0.023 
atmosphere.  In  the  remaining  instruments,  the  average  depression  is 
about  15  millimeters,  or  0.02  atmosphere.  The  real  difficulty  with 
the  capillary  depression  is  due  to  the  fact  that  in  most  tubes  it  varies 
frequently  and  largely  within  short  distances.  In  addition  to  these 
sharp  local  fluctuations,  there  is  nearly  always  a  gradual  increase  or 
diminution  of  the  depression  due  to  a  corresponding  general  change  in 
the  diameter  of  the  bore,  the  diameter  at  one  end  of  the  tube  being 
usually  larger  than  at  the  other. 

Owing  to  the  large  changes  which  may  occur  within  short  distances, 
it  is  necessary  to  determine  the  amount  of  the  capillary  depression  at 
a  great  many  points  in  a  tube.  By  way  of  illustrating  the  importance 
of  doing  so,  the  following  partial  record  of  the  capillary  depressions 
which  were  found  at  different  places  in  one  manometer  is  given.  In 
one  column  of  the  table,  there  are  recorded  the  distances  above  the 
lower  "scratch"  at  which  observations  were  made;  and  in  the  other, 
the  depressions  which  were  found  at  these  points. 

TABLE  2. 


Distance 
above 
scratch. 

Capillary 
depression. 

Distance 
above 
scratch. 

Capillary 
depression 

8.65 

7.92 

117.43 

11.42 

22.70 

10.85 

224.12 

11.18 

47.35 

9.87 

280.30 

11.74 

71.38 

10.04 

361.10 

11.80 

114.28 

10.42 

414.10 

12.14 

A  difference  of  1  millimeter  in  the  capillary  depression  is  equivalent 
to  about  one  calibration  unit  in  determining  the  volume  of  the  nitrogen 
in  the  manometer.  Suppose  now  the  capillary  depression  of  this  tube 
had  been  determined  only  at  nine  points,  beginning  with  the  second 
one,  22.7  millimeters  above  the  scratch.  The  mean  of  the  values 


40 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


is  11.05,  which  number  might  have  been  accepted  as  the  mean  capillary 
depression  of  the  manometer.  But  suppose  when  the  volume  of  the 
nitrogen  in  the  manometer  is  determined,  the  meniscus  stands  8.65 
millimeters  above  the  scratch,  where  the  depression  is  in  reality  only 
7.92  millimeters.  The  error,  if  the  mean  number  11.05  is  used  in 
correcting  for  capillary  depression,  would  be  about  11.05  —  7.92  =  3.13 
calibration  units.  The  whole  of  the  nitrogen  in  this  manometer 
amounts  to  only  400  calibration  units.  The  error  made  in  determining 
the  volume  would  therefore  be  0.78  per  cent.  This  example  of  what 
might  happen  if  the  condition  of  the  tube  at  8.65  millimeters  above 
the  scratch  had  escaped  detection  will  serve  to  convince  one  of  the 
necessity  of  a  detailed  investigation  of  the  capillary  depression  in  tubes 
of  small  bore;  also  of  the  advisability  of  using  manometers  of  large 
capacity,  like  those  seen  in  Figures  19-21,  in  order  to  minimize  errors 
of  capillary  depression  as  well  as  those  of  meniscus. 


FIG.  22. — "Steel  block"  for  the  determination  of  gas  volumes  in  manometers,  fo/r  the  comparison 
of  instruments,  and  for  the  determination  of  capillary  depression. 

(1)  Mercury  reservoir;  (2)  plunger  for  coarse  adjustment  of  pressure;  (3)  plunger  for  fine  adjust- 
ments; (4),  (5),  and  (6)  manometers;  (7),  (8),  (9),  (10),  and  (11)  packing;  (12),  (13),  (14), 
(15),  and  (16)  nuts  for  compression  of  packing. 

Capillary  depression  appears  twice  as  an  important  factor  in  the 
measurement  of  osmotic  pressure:  (1)  in  determining  the  volume  of 
the  nitrogen  under  standard  conditions  of  temperature  and  pressure; 
and  (2)  in  correcting  its  volume  under  an  unknown  pressure,  which 
(i.  e.,  the  osmotic  pressure)  is  a  quotient  of  the  two  volumes. 

An  instrument  much  used  in  the  determination  of  capillary  depres- 
sion, and  also  in  the  comparison  of  manometers,  is  the  " steel  block" 
seen  in  Figure  22.  It  contains  a  reservoir  for  mercury  (1)  and  two 
plungers,  one  of  which  (2)  is  large,  and  the  other  (3)  small.  The  larger 
one  is  employed  for  the  coarser,  and  the  smaller  one  for  the  finer, 
adjustments  of  pressure  in  tubes  4,  5,  and  6.  The  packing  (7,  8,  9,  10, 
and  11),  which  may  be  of  leather  or  rubber,  or  partly  of  both,  is  com- 
pressed in  each  case  between  the  concave  surfaces  of  two  steel  disks 
and  the  required  pressure  is  brought  upon  these  by  means  of  the 


THE    MANOMETERS. 


41 


threaded  plugs  12, 13, 14, 15,  and  16.    The  instrument  has  been  tested 
and  found  to  be  mercury-tight  up  to  350  atmospheres. 

Pure  mercury  only  is  put  into  the  block,  but  it  can  not  be  presumed, 
under  the  prevailing  conditions,  to  maintain  its  purity  unimpaired; 
hence  some  precautions  are  necessary  to  prevent  contamination  of  the 
mercury  in  the  instruments  under  investigation  or  a  fouling  of  the 
glass  walls  of  the  tubes.  The  usual  precaution  is  to  fuse  the  calibrated 
portion  of  the  manometer  to  one  end  of  a  glass  tube  of  nearly  equal 
bore,  which  has  been  bent  to  a  double  U  form.  In  the  intermediate 
limb  a  bulb  is  blown  that 
serves  as  a  reservoir  of 
pure  mercury  for  use  in 
the  manometer  proper. 
Having  filled  the  instru- 
ment with  pure  mercury, 
it  is  fastened  in  place  in  the  steel  block.  The 
arrangement  for  adjusting  the  height  of  the 
mercury  in  the  tube  under  examination  and 
for  determining  capillary  depression  by  differ- 
ence of  level  consists  of  a  glass  tube  having 
an  internal  diameter  of  35  millimeters,  which 
is  connected,  by  means  of  a  rubber  tube,  with 
a  second  glass  tube  occupying  one  of  the  holes 
in  the  steel  block.  In  order  to  render  the 
rubber  tube  sufficiently  rigid,  and  thereby  to 
avoid  unnecessary  oscillations  of  the  mercury 
meniscus,  it  is  tightly  wound  with  several  thick- 
nesses  of  insulating  tape .  The  remaining  hole 
in  the  block  is  usually  occupied  by  a  tube  whose 
capillary  depression  has  been  investigated  in 
great  detail. 

Formerly  it  was  attempted  to  determine 
capillary  depression  by  means  of  comparisons 
with  a  standard,  i.  e.,  by  dispensing  with  the 
wide  tube  mentioned  above  and  inserting  in  one  of  the  holes  of  the 
steel  block  a  tube  whose  capillary  depression  in  every  part  was  known. 
This  is  a  much  more  convenient  method,  but  it  was  abandoned  because 
it  was  found  that  the  errors  of  the  standard  add  themselves  to  those 
of  the  other  instrument.  The  same  difficulty  makes  itself  felt  when 
it  is  attempted  to  compare  one  manometer  with  another.  In  such 
cases  it  is  impossible  to  tell  to  what  extent  the  observed  discrepancy 
is  due  to  the  incorrectness  of  the  values  assigned  to  the  capillary 
depression  of  each  instrument.  It  is  as  likely  to  be  the  sum  as  the 
difference  of  the  two.  In  any  event,  it  is,  of  course,  their  algebraic  sum. 

Another  important  instrument  in  connection  with  the  investigation 
of  manometers  is  the  "tapper "  seen  in  Figure  23.    In  the  measurement 


FIG.  23. 

Electric  hammer  for  tapping 
manometers. 


42       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

of  osmotic  pressure,  the  mercury  has  ample  time  to  adjust  itself,  or 
the  adjustment  is  aided  by  means  to  be  described  hereafter;  but  in 
operations  connected  with  the  determination  of  capillary  depression, 
and  with  the  comparison  and  verification  of  these  instruments,  the  "lag" 
of  the  mercury  must  be  overcome  by  jarring  the  tubes,  and  frequently 
the  tapping  to  which  it  is  necessary  to  subject  tubes  of  small  bore  is 
severe  and  prolonged.  This  is  notably  the  case  in  manometers  in  which 
the  glass  is  not  perfectly  clean  or  has  been  slightly  roughened  by  the 
reagents  employed  in  cleansing  it. 

The  construction  of  the  " tapper"  is  so  obvious  that  it  is  only  neces- 
sary to  notice  two  or  three  of  its  features.  It  is  strongly  inclined, 
when  in  operation,  to  move  away  from  the  tube  which  the  " hammer" 
is  striking.  Hence  the  base  is  made  of  lead,  for  the  sake  of  greater 
weight,  and  is  mounted  upon  three  very  sharp-pointed  pegs,  which 
sink  somewhat  into  the  wood  on  which  the  instrument  rests.  The 
hammer  is  covered  with  rubber  or  leather  to  prevent  the  possible 
shattering  effect  of  its  blows.  The  tapper  is  connected,  by  means  of  a 
flexible  wire  cord,  through  the  battery,  with  a  portable  push-button 
which  is  held  in  the  hand  of  the  observer  behind  the  cathetometer,  who 
can  therefore  at  any  time  hammer  the  tube  without  removing  his  eye 
from  the  telescope. 

During  the  determination  of  capillary  depression  and  other  operations 
which  are  connected  with  the  preparation  of  manometers  for  use,  the 
instruments  must  be  kept  at  constant  temperature.  Otherwise  the  all- 
important  meniscus  is  continually  changing  its  form,  to  the  great  con- 
fusion of  the  observer.  The  first  effective  device  for  the  maintenance 
of  temperature  was  the  so-called  "manometer  house,"  which  is  seen— 
stripped  of  its  coverings — in  Figure  24  and  Plate  II.  It  was  in  this 
that,  for  several  years,  all  experiments  on  manometers,  except  calibra- 
tion in  the  horizontal  position,  were  carried  out.  The  "house"  con- 
tains the  "steel  block,"  the  "brass  block" — to  be  described  later — the 
"tapper,"  a  meter  scale,  a  thermostat  for  the  regulation  of  temperature, 
electric  heaters  (lamps),  and  a  fan  motor,  all  of  which  will  be  recognized 
in  the  figures.  The  shelf  (Figure  24),  on  which  rest  the  various  instru- 
ments, is  supported  by  heavy  steel  brackets  (not  shown  in  the  figure), 
which  are  bolted  to  the  heavy  masonry  wall  behind,  and  afford  a  satis- 
factory degree  of  stability.  At  each  end  of  the  shelf,  a  space  5  centi- 
meters wide  is  left  for  the  passage  of  air.  Lamps  are  employed  as  the 
source  of  heat,  for  the  reason  that  they  heat  up  and  cool  down  more 
quickly  than  other  electric  heating  appliances.  They  are  under  the 
control  of  the  thermostat  seen  in  the  upper  part  of  the  house.  The  fan 
is  stationed  before  a  hole  of  equal  diameter  in  the  partition  2.  By 
means  of  it,  the  air,  heated  by  the  lamps,  is  kept  in  continuous  circu- 
lation over  all  the  instruments.  The  temperature  which  is  maintained 
in  the  compartment  is  always  higher  by  a  few  degrees  than  the  highest 
temperature  of  the  room  in  which  it  is  located. 


MORSE 


PLATE  2 


THE    MANOMETERS. 


43 


The  remaining  features  are  better  seen  in  the  photograph  (Plate  2), 
where  the  manometer  house  is  represented  with  the  plate-glass  front 
removed.  The  end  to  the  right  and  the  top  of  the  house  are  also  of 


Fio.  24. 
"Manometer  house"  for  the  calibration  and  comparison  of  instruments,  etc. 

glass,  though  the  latter  is  usually  covered  with  a  thick  woolen  pad  and 
the  former  with  a  flannel  curtain.  The  front  is  also  provided  with  a 
flannel  curtain  (not  seen  in  the  figure),  which  may  be  parted  at  con- 


44 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


venient  places  for  observation.  The  frame  for  the  glass  at  the  top  is 
removable  to  provide  for  the  extension  of  the  house  upward  when 
very  long  tubes  are  to  be  accommodated.  The  various  windows  and 
doors  are  made  to  close  tightly  against  rubber  cushions,  or  the  cracks 
between  them  and  the  frame- 
work are  covered  with  surgeons' 
tape.  The  tubes  through  which 
the  wires  enter  the  house  are, 
however,  left  more  or  less  open 
to  provide  for  equalization  of 
atmospheric  pressure. 

During  the  past  year  or  two, 
the  more  exacting  parts  of  the  in- 
vestigation of  manometers  have 
been  carried  out  in  the  bath  seen 
in  Figure  45.  This  bath  is  am- 
ple enough  to  accommodate  the 
steel  block,  the  tapper,  and  all 
other  accessories  required  for  a 
determination  of  capillary  de- 
pression, or  of  nitrogen  volume, 
and  for  the  comparison  of  mano- 
meters; and  in  it  temperatures 
can  be  maintained  for  long  per- 
iods which  are  constant  to  0.01°. 

Plate  2  shows  the  type  of 
cathetometer  used,  and  under  it 
a  specimen  of  the  devices  by 
means  of  which  the  requisite 
degree  of  steadiness  for  all  the 
instruments  is  secured,  notwith- 
standing their  location  in  the 
third  story  of  the  laboratory. 
The  foundation  for  the  catheto- 
meter consists  of  two  heavy 
wooden  brackets.  One  end  of 
the  horizontal  timbers  is  buried 
in  the  thick  brick  wall  behind 
the  house,  while  the  descending 
timbers  pass  through  the  floor 
and  enter  the  same  wall  in  the 
room  below.  There  is  nowhere  contact  with  a  floor  or  with  a  parti- 
tion wall.  Two  such  brackets  are  required  for  a  cathetometer  and 
three  for  a  bath. 

In  Figure  25  is  shown  an  improved  arrangement  for  fine  adjustment 
of  the  height  of  the  telescope,  and  for  reading  fractional  parts  of  a  milli- 


FIG.  25. — Improvement  in  cathetometers  for  the  fine 
adjustment  of  the  telescope,  which  also  serves  as  a 
substitute  for  the  micrometer  eye-piece. 

(a)  Set-collar  with  upper  and  thicker  end  threaded — 
thread  1  millimeter  pitch;  (b)  nut  running  over 
(a),  and  graduated  in  hundredths;  (c)  ring  attached 
to  sleeve  carrying  telescope,  and  resting  on  (6). 


THE    MANOMETERS.  45 

meter  on  the  graduated  scale.  It  consists  of  a  sliding  collar  (a),  on  the 
upper  and  heavier  end  of  which  has  been  cut  a  thread  of  1  millimeter 
pitch.  Over  this  runs  the  internally  threaded  collar  (6) ;  and  upon  b 
rests  the  sleeve  (c)  on  which  is  mounted  the  telescope.  The  collar  (6)  is 
graduated  in  100  equal  parts,  while  c  has  engraved  upon  it  a  vertical 
zero  line.  One  entire  revolution  of  b  corresponds  therefore  to  a  rise  or 
descent  of  1  millimeter  in  the  telescope,  and  its  movements  up  and 
down  can  be  read  directly  to  hundredths,  and  estimated  to  thousandths 
of  a  millimeter.  The  device  is  a  substitute  for  the  usual  micrometer 
eye-piece  on  the  telescope,  and  has  the  advantage  over  the  latter  that 
it  is  not  necessary  to  have  a  precisely  fixed  distance  between  the  eye- 
piece and  the  graduated  scale.  A  second  advantage,  considered  as  a 
means  of  elevating  and  lowering  the  telescope,  is  that  the  whole  weight 
of  the  telescope  and  its  balanced  carriage  is  uniformly  distributed  upon 
the  top  of  the  collar  (6)  and  ultimately  upon  the  upper  side  of  the  thread. 
Hence,  when  the  collar  is  turned,  there  is  neither  any  of  that  "lurching" 
of  the  telescope  which  is  so  offensive  in  the  older  arrangements,  nor  any 
"back  lash"  on  the  thread. 

THE  FILLING  OF  THE  MANOMETER. 

When  the  manometer  has  been  calibrated  and  the  value  of  the  menis- 
cus correction  ascertained,  and  the  extent  of  the  capillary  depression  has 
been  determined  at  a  great  many  points,  it  is  joined  to  the  uncalibrated 
portion  of  the  instrument  and  filled  with  nitrogen. 

Originally  the  manometers  were  filled  with  purified  and  dried  air,  but 
it  was  found  that,  however  pure  the  mercury  in  them  might  be,  the 
volume  of  the  included  air  slowly  diminished.  At  first  it  was  suspected 
that  this  diminution  in  the  volume  might  be  only  apparent;  in  other 
words,  that  the  capacity  of  the  manometers  was  increasing  under  the 
pressures  to  which  the  gas  was  subjected.  To  test  this  suspicion,  long 
columns  of  mercury  were  placed  in  calibrated  tubes,  like  those  used  for 
manometers,  between  columns  of  air;  and  these  were  then  subjected  to 
pressures  equal  to  the  highest  osmotic  pressures  which  were  being 
measured.  The  purpose  was  to  discover  whether  the  columns  of  mer- 
cury, under  such  treatment,  diminished  sensibly  in  length — either 
temporarily  or  permanently.  The  results  were  wholly  negative.  It 
was  therefore  concluded  that  the  observed  decrease  in  the  volume  of 
the  imprisoned  air  must  be  due  to  the  action  of  the  oxygen  on  the  mer- 
cury, though  no  fouling  of  the  glass,  such  as  would  be  expected  from 
the  presence  of  oxides,  had  been  noticed.  A  third  possible  explanation, 
namely,  that  in  the  course  of  the  movements  of  the  mercury  back  and 
forth  some  of  the  gas  had  been  "rubbed  out"  of  the  tubes,  was  not  seri- 
ously considered.  If  the  loss  in  volume  of  gas  was  due  to  the  disap- 
pearance of  oxygen,  the  obvious  remedy  was  to  fill  the  manometer  with 
nitrogen.  The  remedy  was  so  complete  that,  after  years  of  use,  no 
change  in  the  volume  of  that  gas  in  the  manometers  has  been  observed. 


46 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


The  nitrogen  used  in  the  manometers  is  obtained  by  passing  air  first 
through  an  alkaline  solution  of  pyrogallol,  and  then,  in  the  order  named, 
over  heated  copper  oxide,  heated  copper,  heated  copper  oxide,  cal- 
cium chloride,  fused  potassium 
hydroxide,  and  resublimed  phos- 
phorus pentoxide.  The  glass 
tubes  containing  the  dry  reagents 
are  all  connected  with  each  other 
and  with  the  receptacle  for  the 
nitrogen  by  fusing  the  ends  to- 
gether. 

The  arrangement  of  apparatus 
for  filling  the  manometers  is 
shown  in  Figure  26.  The  method 
of  filling,  because  of  its  complexity 
and  the  difficulty  of  some  of  its  parts,  will  be 
described  in  considerable  detail. 

A  is  the  reservoir  in  which  the  purified 
nitrogen  is  stored  up,  and  from  which  the 
manometers  are  filled.  The  unlettered  stop- 
cock at  the  top  is  that  through  which  the 
gas,  after  purification,  enters  the  reservoir. 
B  is  the  calibrated  and  thoroughly  cleansed 
manometer  which  is  to  be  filled  and  closed, 
and  C  is  an  arrangement  for  filling  and 
emptying  the  manometer.  B  is  joined  to 
A,  at  d,  by  fusing  together  the  ends  of  the 
glass  tubes;  and  to  C,  at  E,  by  means  of 
rubber  tubing.  The  mercury  in  C  is  sep- 
arated from  that  in  the  manometer  by  the 
air  which  nearly  fills  the  wide  tube  below  E. 
In  this  way,  the  mercury  in  C,  which  may 
be  impure  from  its  contact  with  rubber  tub- 
ing, is  prevented  from  entering  the  manom- 
eter and  contaminating  the  very  pure  mer- 
cury with  which  that  instrument  is  filled. 
This  air  also  plays  an  important  role  when 
the  manometer  is  closed. 

Before  joining  the  manometer  B  to  A 
and  C,  its  lower  end  is  immersed  in  pure 
mercury  and,  with  the  instrument  in  an 
inclined  position,  gentle  suction  is  applied 
until  the  two  bulbs  are  filled  as  nearly  as 
may  be  with  mercury.  Owing  to  the  presence  of  one  or  more  traps, 
some  air  will  be  left  in  the  bulbs,  and  this  must  be  expelled  by 
bringing  the  instrument  into  the  vertical  position  and  forcing  the 


FIG.  26. — Arrangement  for  filling 
manometers  with  nitrogen. 

(A)  Nitrogen  reservoir;  (B)  cali- 
brated manometer;  (C)  air  cham- 
ber to  separate  mercury  in  (t) 
from  pure  mercury  in  manometer; 
(E)  connector  between  (B)  and 
(C);  (/)  and  (t)  mercury  reser- 
voirs; (k)  and  (h)  two-way  stop- 
cocks; (f)  and  (0)  vents. 


THE   MANOMETERS.  47 

mercury  in  the  other  direction.  When  the  bulbs  and  more  or  less 
of  the  tube  B  have  been  filled,  and  the  junctions  at  d  and  E  have 
been  made,  the  manometer  is  repeatedly  washed  out  with  air  which 
has  been  dried  by  resublimed  phosphorus  pentoxide.  For  this  pur- 
pose, by  lowering  the  reservoir  (i),  the  dried  air  is  admitted  through 
the  stopcock  at  the  top,  and  likewise  through  /,  which  is  also  provided 
with  a  drying  tube.  By  raising  i,  it  is  again  expelled,  mostly  through 
g,  but  partly  through  /. 

The  next  step,  after  drying  the  manometer,  is  to  fill  it  with  nitrogen 
from  the  reservoir  (A).  The  reservoir  (i)  is  raised  and  the  air  in  the 
manometer  is  expelled  through  g  until  the  mercury  column  reaches  j, 
when  the  stopcock  (h)  is  closed,  and  a  small  quantity  of  mercury  is 
driven  into  the  side  tube  (/).  This  is  the  mercury  which  is  afterwards 
to  occupy  the  upper  end  of  the  closed  manometer.  The  rubber  tube 
connecting/  with  its  drying  tube  is  tightly  closed  and  the  air  remaining 
between  j  and  the  stopcock  (h)  is  expelled  through  h,  care  being  taken 
not  to  allow  the  mercury  quite  to  reach  the  stopcock,  lest  it  should  be 
contaminated  by  some  of  the  lubricant  on  the  latter.  Some  of  the 
nitrogen  in  A  is  repeatedly  wasted  through  the  stopcock  at  the  top  and 
through  g  in  order  to  remove  any  air  still  remaining  in  the  upper  part 
of  the  apparatus.  Then  by  lowering  i  or  raising  I,  with  stopcock  k  open, 
the  manometer  is  filled  with  nitrogen.  This  is  wasted  through  g,  and  the 
manometer  is  again  filled  from  A,  and  the  operation  of  filling  and  empty- 
ing it  is  repeated  as  many  times  as  may  be  thought  necessary. 

When  the  manometer  has  been  filled  with  nitrogen  for  the  last  time, 
the  reservoir  (i)  is  adjusted  to  the  right  level,  and  the  gas  is  placed 
under  a  slight  over  pressure  by  raising  I.  The  stopcock  (h)  is  opened 
and  then  quickly  closed.  This  leaves  the  nitrogen  in  the  manometer 
under  a  pressure  equal  to  that  of  the  atmosphere. 

By  gently  pinching  the  rubber  tube  which  closes/,  a  little  mercury  is 
forced  out  of  the  side  tube  into  the  vertical  one  between  j  and  d.  If  it 
breaks  into  globules  at  j,  they  are  reunited  at  d  by  tapping  the  tube. 
The  mercury  thus  transferred  does  not  enter  the  manometer,  because 
of  its  small  bore. 

The  reservoir  (i)  is  now  lowered  until  all  the  mercury  collected  at  d 
has  been  drawn  into  the  manometer  to  some  convenient  distance  below 
that  point,  when  the  glass  at  d  is  softened  in  the  blowpipe  flame  and  the 
manometer  is  detached,  but  so  as  to  leave  both  tubes  sealed. 

The  glass  at  the  detached  end  of  the  manometer  is  again  softened  in 
the  flame  and  then  drawn  out  to  an  exceedingly  fine  capillary  tube, 
which  is  afterwards  filled  with  mercury  by  raising  i.  Finally  the  capil- 
lary is  closed  in  the  flame,  and  the  walls  are  thickened  under  slightly 
diminished  pressure.  Care  must  be  taken,  in  closing  the  manometer, 
not  to  convert  any  considerable  amount  of  the  mercury  into  vapor, 
and  to  heat  the  glass  so  uniformly  that  the  vapor  which  is  necessarily 


48  OSMOTIC   PRESSURE   OF   AQUEOUS    SOLUTIONS. 

formed  can  not  recondense  until  the  operation  of  closing  is  finished. 
Otherwise  the  violent  agitation  of  the  mercury,  due  to  rapid  vaporiza- 
tion and  condensation,  is  apt  to  shatter  the  tube.  When  closed,  no 
bubble  of  air  should  be  discernible  at  the  top  of  the  short  mercury  col- 
umn. First  attempts  at  closing  usually  fail  in  this  respect,  but  after  a 
little  practice,  one  is  able  to  perform  the  operation  with  perfect  success. 
The  short  column  of  mercury  in  the  upper  end  of  the  manometer  has 
a  twofold  purpose.  It  prevents,  during  the  closing  of  the  instrument, 
any  contamination  of  the  nitrogen  with  air  or  with  the  combustion 
products  of  the  flame;  and  it  fills  up  all  that  portion  of  the  instrument 
whose  caliber  may  have  been  altered  to  an  unknown  extent  by  heating. 

DETERMINATION  OF  THE  VOLUME  OF  THE  NITROGEN. 

For  this  purpose,  the  manometer  is  placed  in  the  steel  block  within 
the  bath  (Figure  45),  and  the  pressure  upon  the  gas  is  regulated  by  the 
device  used  in  the  determination  of  capillary  depression,  i.  e.,  a  glass 
tube  having  an  internal  diameter  of  35  millimeters,  which  is  connected 
with  the  steel  block  by  means  of  a  flexible  tube.  Formerly  it  was 
attempted  to  use  a  stationary  "side"  tube.  This  consisted  of  a  short 
piece  cut  from  the  same  tube  as  the  manometer  itself  and,  like  the  ma- 
nometer, it  was  fixed  rigidly  in  the  block,  the  pressure  being  regulated  by 
the  plungers.  The  practice  was,  however,  based  on  the  mistaken  assump- 
tion that  in  any  given,  fairly  good  tube  the  capillary  depression  is  nearly 
uniform  throughout.  It  was  discontinued  when  it  was  discovered  that 
the  best  tubes  we  could  obtain  were  very  uneven  in  this  respect. 

The  volume  of  the  nitrogen  is  determined  under  a  number  of  different 
pressures,  all  of  them,  of  course,  quite  near  that  of  the  atmosphere.  To 
determine  it  under  high  pressures,  it  is  necessary  to  employ  another 
closed  manometer — a  so-called  "standard  manometer."  There  is,  how- 
ever, the  same  objection  to  the  employment  of  standard  manometers  as 
to  the  use  of  narrow  side  tubes  in  the  determination  of  capillary  depres- 
sion and  of  gas  volumes,  the  objection,  namely,  that  all  the  errors  of 
both  tubes — principally  of  capillary  depression — are  charged  to  the  tube 
under  investigation. 

Sometimes,  in  order  to  increase  the  quantity  of  the  gas  in  the  ma- 
nometer, more  than  the  calibrated  portion  of  the  tube  has  been  filled 
with  nitrogen.  This  was  frequently  done  before  the  introduction  of 
manometers  with  large  reservoirs  of  known  capacity  (Figure  19,  etc.). 
In  such  cases  the  use  of  a  standard  manometer  could  not  be  avoided. 

For  the  comparison  of  one  manometer  with  another,  the  steel  block 
and  also  the  " brass  block"  seen  in  Figure  27  are  used.  The  latter 
does  not  differ  in  construction  from  the  former,  except  in  the  means 
for  fixing  the  tubes  in  their  places.  The  arrangements  employed  for 
that  purpose  are  identical  with  those  used  in  joining  the  cells  and  the 
manometers.  Some  other  liquid  than  mercury — either  water  or  a 
solution — is  used  in  the  brass  block.  v 


THE   MANOMETERS. 


49 


When  any  operation  is  to  be  performed  with  a  manometer  which 
might  endanger  the  calibrated  portion,  or  contaminate  the  mercury 
in  it,  or  foul  the  walls,  the  instrument  is  cut  into  two  parts,  the  point 
of  severance  being  usually  between  the  bulbs,  when  that  is  practicable. 
If  necessary,  another  piece  of  suitable  form  is  then  attached  to  the 
manometer,  e.  g.,  as  when  the  instrument  is  to  be  placed  in  the  steel 
block.  Afterwards  the  detached  portion  is  restored  to  its  place. 


FIG.  27. — "Brass  block." 

Construction  like  that  of  "steel  block"  (see  Figure  22),  except  the  manometer  attachments,  which 
are  like  those  used  with  the  cells. 

In  Figure  28  is  seen  an  instrument  much  used  in  the  manipulation 
of  the  manometers.  The  manner  of  its  use  will  be  best  illustrated  by 
describing  a  few  of  the  operations  in  which  it  is  most  frequently  employed. 

1.  Suppose  the  whole  instrument  (Figure  18  or  19),  except  the  space 
occupied  by  the  nitrogen,  is  filled  with  mercury,  and  it  is  necessary 
to  cut  the  tube  between  the  bulbs  1  and  2,  either  for  the  purpose  of 
replacing  the  detached  piece  by  another  of  similar  form,  or  by  a  simpler 
piece  of  glass  tubing.  The  rubber-covered  cone  which  is  usually  upon 
the  open  end  of  the  manometer,  or  a  sharply  sloping  stopper  through 
which  the  end  has  been  passed,  is  placed  in  the  cup  1.  The  air  is  then 
exhausted  through  a  rubber  tube  attached  to  the  stem  at  2.  When  a 
sufficient  quantity  of  mercury  has  been  drawn  into  the  cup,  the  whole 
arrangement  is  tipped  backwards  until  the  end  of  the  manometer  is 
exposed.  Air  is  then  cautiously  readmitted  to  fill  the  space  in  the 
instrument  which  was  previously  occupied  by  the  mercury  removed. 


50       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

2.  Suppose  the  open  end  of  the  manometer,  e.  g.,  to  the  middle  of 
bulb  No.  1,  is  filled  with  air  and  it  is  desired  to  replace  it  with  mercury. 
A  quantity  of  mercury  is  poured  into  the  cup,  the  manometer  is  inserted 
and,  with  the  instrument  tipped  so  as  to  expose  the  open  end,  the  air 
is  exhausted  until  the  mercury  begins  to  run  out.  On  bringing  the 
manometer  again  to  the  upright  position,  so  as  to  immerse  the  open 
end,  and  readmitting  air,  the  mercury  flows  into  the  tube  to  replace 
the  air  which  has  been  withdrawn. 


FIG.  28. 

Apparatus  used  in  emptying,  filling, 
and  cleansing  the  uncalibrated 
portion  of  the  manometers. 


(1)  Reservoir  for  solutions  or 
mercury;  (2)  place  for  at- 
taching tubes  containing 
absorbents. 

3.  Suppose,  again,  the  manometer  has  been  used  in  a  measurement 
of  pressure,  and  the  open  end — perhaps  also  a  small  portion  of  bulb 
No.  1 — is  filled  with  the  solution.  Before  the  instrument  can  be  used 
for  another  experiment,  this  must  be  removed  and  replaced,  either  by 
mercury  or  by  some  of  the  solution  whose  pressure  is  to  be  determined. 
The  necessary  manipulation  is  as  follows:  (1)  the  old  solution  is 
removed  and  replaced  by  ah*;  (2)  the  air  is  replaced  by  the  new  solu- 
tion, and  this,  in  turn,  is  replaced  in  succession  by  other  portions  of  the 
same  solution,  until  there  is  no  danger  that  the  concentration  of  the 
new  solution  will  be  affected  by  the  older  one.  If  mercury  is  to  be 
substituted  for  a  solution,  the  tube  must  be  washed  and  dried  before 
introducing  it.  In  this  case,  portions  of  the  wash  liquids — water, 
alcohol,  and  redistilled  ether — are  introduced  and  removed  in  exactly 
the  same  manner  as  when  one  solution  is  to  be  substituted  by  another. 
The  final  drying  of  the  manometer  is  accomplished  by  attaching  a 
tube  containing  drying  agents  or  absorbents  to  the  stem  (2)  and  alter- 
nately exhausting  and  readmitting  air.  When  manometers  of  the 
forms  seen  in  Figures  20  and  21  are  to  be  dealt  with,  the  instrument 
(Figure  28)  is  attached  to  the  vent.  The  manipulation  is  then  more 
complex  but  not  less  effective. 

The  time  consumed  in  preparing  a  manometer  for  the  measurement 
of  osmotic  pressure  is  usually  about  one  month. 


CHAPTER  III. 
THE  REGULATION  OF  TEMPERATURE. 

THERMOMETER  EFFECTS. 

Because  of  certain  obvious  analogies  between  a  closed  osmotic  cell 
and  a  sensitive  thermometer,  the  name  "thermometer  effects"  has  been 
given  to  a  large  group  of  exceedingly  troublesome  manifestations  which 
follow  even  slight  fluctuations  in  bath  temperature.  The  name  is 
appropriate  only  in  a  very  restricted  sense.  The  phenomena  thus 
classified  are  complex  and  often  they  are  difficult  to  analyze  satisfac- 
torily. To  understand  them,  one  needs  to  keep  constantly  in  mind 
three  fundamental  facts:  (1)  That  the  capacity  of  the  closed  osmotic 
cell  is  a  nearly  fixed  quantity;  (2)  that  every  change  in  the  volume  of 
its  contents — due  to  rise  or  fall  of  temperature — is  followed  by  a  dis- 
charge or  intake  of  solvent  through  the  membrane,  both  of  which  acts 
also  modify  the  volume  and  the  osmotic  pressure  of  the  solutions;  and 
(3)  that  the  passage  of  the  solvent  through  the  membrane,  in  either 
direction,  is  usually  a  much  slower  process  than  the  changes  in  the 
volume  of  the  cell  contents  which  result  from  fluctuations  of  tempera- 
ture. The  first  and  second  of  the  enumerated  facts  are  obviously  true, 
but  the  third,  which  is  responsible  in  largest  measure  for  the  complex 
and  often  perplexing  results,  can  be  learned  only  by  experience. 

The  four  elementary  fluctuations  of  temperature  and  their  conse- 
quences will  be  considered: 

(1)  The  temperature  of  the  bath  (previously  constant)  rises  and 

becomes  again  constant  at  the  higher  level. 

(2)  After  rising,  it  falls  again  to  the  original  level. 

(3)  The  temperature  of  the  bath  (previously  constant)  falls  and 

remains  constant  at  the  lower  level. 

(4)  After  falling,  it  rises  again  to  the  original  level. 

The  question  to  be  answered  is,  what  changes  in  cell  pressure  will  the 
observer  at  the  telescope  see  in  consequence  of  the  temperature  fluc- 
tuations enumerated  above?  For  convenience,  all  positive  pressure 
in  the  cell  which  is  not  osmotic  will  be  called  mechanical,  and  the  sum  of 
the  two  will  be  spoken  of  as  the  total  pressure. 

1.  The  conditions  which  are  supposed  to  prevail  are  as  follows:  The 
cell  contains  a  solution  of  known  concentration,  the  temperature  is  con- 
stant, and  the  solution  is  exhibiting  its  true  osmotic  pressure  only. 
Subsequently  the  temperature  rises  and  becomes  constant  again  at  the 
higher  level.  This  is  the  simplest  of  the  four  cases  previously  men- 
tioned. The  volume  of  the  liquids  in  the  cell — the  mercury  in  the 
manometer  and  the  solution — and  the  tension  of  the  gas  in  the  manom- 

51 


52        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

eter  increase.  The  total  pressure  is  now  the  sum  of  the  osmotic  pres- 
sure of  the  solution  and  a  considerable  mechanical  pressure  due  to  the 
expansion  of  the  liquid  contents  of  the  cell.  In  consequence  of  this 
over-pressure — the  difference  between  the  total  and  the  osmotic  pres- 
sures— the  gas  in  the  manometer  is  compressed  to  a  smaller  volume,  and 
the  mercury  meniscus  is  seen  to  rise  and  finally  to  attain  to  a  maximum 
height.  Simultaneously  with  the  expansion  of  the  liquids  in  the  cell, 
there  is,  in  consequence  of  the  over-pressure,  a  very  slow  outward  dis- 
charge of  the  solvent  through  the  membrane,  the  effect  of  which  is  two- 
fold. First,  there  is  a  reduction  in  the  volume  of  the  solution  which 
reduces  the  mechanical  pressure;  and,  second,  an  increase  in  osmotic 
pressure  due  to  the  increasing  concentration  of  the  solution.  The  two 
effects  are  of  a  mutually  compensatory  character,  but  they  are  not  equal 
in  their  opposite  influences  upon  the  magnitude  of  the  pressure  in  the 
cell.  Hence  the  meniscus  does  not  remain  at  the  highest  point  reached 
by  it,  but  sinks  again  and  becomes  stationary  at  a  lower  level  only  when 
the  mechanical  pressure  has  wholly  disappeared  and  the  only  pressure 
in  the  cell  is  the  osmotic  pressure  of  a  solution  more  concentrated  than 
the  original  one.  To  recapitulate,  the  meniscus,  in  the  case  under  con- 
sideration, takes  three  positions  in  the  manometer,  which  may  be  called, 
in  the  order  of  their  relative  heights,  the  lowest,  the  intermediate,  and 
the  highest.  The  first  and  the  second  of  these  correspond  to  the  true 
osmotic  pressures  of  two  solutions  of  different  concentration,  while  the 
third  is  temporary  and  corresponds  to  the  sum  of  an  unknown  mechan- 
ical pressure  and  an  osmotic  pressure  of  which  it  can  only  be  said  that  it 
is  higher  than  the  osmotic  pressure  of  the  more  dilute  and  lower  than 
that  of  the  more  concentrated  solution. 

It  will  be  seen  that  the  maximum  height  to  which  the  meniscus  will 
temporarily  attain  depends  upon  both  the  magnitude  and  the  rate  of 
the  rise  in  temperature,  while  its  final  position  is  determined  solely  by 
the  former.  In  other  words,  a  rapid  rise  in  temperature  always  pro- 
duces a  larger  thermometer  effect  than  a  slow  one.  It  will  be  seen  also 
that  the  magnitude  of  the  thermometer  effect  in  question,  when  trans- 
lated into  pressure,  depends  in  large  measure  upon  the  volume  of  the 
nitrogen  in  the  manometer. 

2.  If,  after  a  rise,  the  temperature,  instead  of  becoming  constant, 
again  sinks  to  its  original  level,  a  more  complicated  series  of  changes  in 
cell  pressure  is  observed.  The  cause  of  the  increased  complexity  of  the 
situation  is  the  falling  temperature  which  may  begin  to  operate  before 
or  after  the  meniscus  has  reached  its  greatest  height.  If  it  begins 
before,  the  meniscus  will  evidently  not  rise  so  high  as  it  otherwise 
would.  For  present  purposes,  let  it  be  supposed  that  the  fall  in  tem- 
perature sets  in  immediately  after  the  meniscus  has  reached  the  highest 
point  in  its  ascent,  i.  e.,  when  the  greatest  pressure  has  been  developed 
in  the  cell.  Up  to  this  point,  then,  the  conditions  are  identical  with 


THE   REGULATION   OF   TEMPERATURE.  53 

those  in  the  preceding  case.  But  there  is  now  a  falling  instead  of  a 
stationary  temperature.  The  elements  of  the  situation  are  (a)  an  over 
or  mechanical  pressure  in  the  cell  due  to  a  previous  rise  in  temperature, 
and  (6)  a  falling  temperature.  The  consequences  of  (a)  are : 

(1)  An  increase  in  osmotic  pressure,  due  to  the  concentration  of  the 

solution  which  follows  the  expulsion  of  solvent. 

(2)  A  decrease  in  mechanical  pressure,  due  to  the  smaller  volume 

of  the  solution  after  expulsion  of  solvent. 
The  consequences  of  (b)  are : 

(3)  A  decrease  in  mechanical  pressure,  due  to  the  diminishing 

volume  of  the  cell  contents. 

(4)  A  decrease  in  osmotic  pressure,  due  to  lower  temperature. 

(5)  A  decrease  in  osmotic  pressure,  due  to  dilution  of  the  solution 

through  intake  of  solvent. 

(6)  An  increase  of  pressure  within  the  cell,  due  to  the  increase  in 

the  volume  of  the  solution  through  intake  of  solvent. 
Of  the  effects  enumerated  above,  (1)  and  (6)  are  positive,  i.  e.,  they 
tend  toward  the  maintenance  or  increase  of  pressure  in  the  cell.  In  the 
same  sense,  (2),  (3),  (4),  and  (5)  are  negative.  The  amount  of  over  or 
under  pressure  in  the  cell  at  any  given  moment  is,  of  course,  the  alge- 
braic sum  of  all  these  effects.  By  "over"  and  "under"  pressure  is 
meant  the  difference  between  the  actual  pressure  in  the  cell  at  any  time 
and  the  true  osmotic  pressure  of  the  solution  at  the  original  temper- 
ature, i.  e.,  before  the  rise  and  subsequent  fall  of  temperature.  One 
would  expect,  perhaps,  that  the  sum  of  the  "over"  and  "under"  pres- 
sures would  become  zero  when  the  bath  had  recovered  its  original  tem- 
perature. In  other  words,  that  the  meniscus  would  stop  in  its  descent 
and  become  stationary,  when  the  pressure  in  the  cell  is  equal  to  the  true 
osmotic  pressure  of  the  solution  at  the  original  and  now  constant  tem- 
perature. But  this  is  by  no  means  the  case.  It  continues  to  descend, 
and,  before  coming  to  a  rest,  may  go  far  below  the  level  which  corre- 
sponds to  the  osmotic  pressure  of  the  original  solution  at  the  given 
temperature.  Here  again  the  reason  for  the  apparently  anomalous 
conduct  of  the  cell  is  to  be  found  in  the  fact  that  changes  in  volume,  due 
to  fluctuations  of  temperature,  are  accomplished  more  quickly  than  the 
migrations  of  solvent  through  the  membrane  which  follow  such  fluctua- 
tions. Having  reached  the  lowest  point  in  its  descent,  the  meniscus 
rises  again  and  finally  comes  to  rest  at  its  original  level,  i.  e.,  at  the  level 
which  corresponds  to  the  true  osmotic  pressure  of  the  original  solution 
at  the  original  temperature.  The  upward  movement  of  the  meniscus 
in  the  final  return  to  its  first  position  is  the  resultant  of  two  opposite 
effects  of  the  intake  of  solvent:  (1)  the  increasing  volume  and  (2)  the 
decreasing  osmotic  pressure  of  the  solution.  To  recapitulate :  If,  after 
a  rise,  the  bath  recovers  its  original  temperature,  the  meniscus  first 
ascends  to  a  point  whose  elevation  above  the  original  position  depends 


54        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

(1)  upon  the  magnitude  and  rate  of  the  rise  in  temperature,  and  (2)  upon 
the  time  at  which  the  fall  in  temperature  sets  in  and  the  rate  of  the 
fall.     The  meniscus  then  descends  to  a  point  below  its  original  level. 
The  distance  between  the  two  positions  depends  (1)  upon  the  magni- 
tude of  the  upward  displacement  and  (2)  the  rate  at  which  the  bath 
recovers  its  original  temperature.     Finally,  the  meniscus  ascends  to  its 
first  place. 

The  third  and  fourth  situations  are  not  more  simple  than  the  first 
and  second,  but  enough  has  already  been  said  for  the  present  purpose, 
which  is  merely  to  emphasize  the  complex  nature  of  thermometer  effects. 
Hence  in  the  remaining  cases  the  movements  of  the  meniscus  only  will 
be  stated. 

3.  The  conditions  are  as  follows:  The  cell  contains  a  solution  of 
known  concentration  which  is  exhibiting  its  true  osmotic  pressure  at 
a  given  constant  temperature  when  a  fall  in  temperature  occurs. 
Afterwards  the  temperature  becomes  constant  at  a  lower  level.     The 
movements  of  the  meniscus  which  are  observed  are  :  (1)  A  fall  (usually 
quite  rapid)  to  a  point  below  the  position  which  it  will  finally  take,  and 

(2)  a  rise  to  some  intermediate  point  at  which  it  becomes  stationary. 
The  final  position  corresponds  to  the  true  osmotic  pressure,  at  the  given 
lower  temperature,  of  a  permanently  diluted  solution.     The  difference 
between  the  lowest  and  final  positions  of  the  meniscus  will  depend 
upon  the  magnitude  of  the  fall  in  temperature  and  upon  its  rate  as 
compared  with  that  of  intake  of  solvent. 

The  movements  of  the  meniscus  are  sometimes  less  simple  than  stated 
above,  since  at  times  one  observes  an  extra  excursion  of  the  meniscus, 
i.  e.,  it  falls  to  its  lowest  level  and  then  rises  to  a  point  above  the  position 
which  it  finally  takes. 

4.  If,  after  the  fall,  the  temperature  rises  and  becomes  constant 
again  at  the  original  level,  which  is  the  most  frequent  case,  the  move- 
ments observed  are  as  follows :  The  meniscus  falls  to  its  lowest  position, 
then  rises  to  one  higher  than  it  had  originally,  and  finally  sinks  to  the 
place  from  which  it  started.     Here  again  an  extra  excursion  of  the 
meniscus  is  sometimes  observed,  namely,  a  second  one  to  a  point  below 
its  final  position.     When  the  meniscus  has  finally  recovered  the  position 
from  which  it  first  started,  we  have  again,  of  course,  the  true  osmotic 
pressure  of  the  original  solution  at  the  original  temperature. 

The  "extra"  excursions  of  the  meniscus  mentioned  under  3  and  4, 
as  well  as  certain  other  anomalies  not  mentioned,  are  probably  due  to 
temporary  inequalities  of  concentration  in  the  solution — to  the  fact, 
namely,  that  when  solvent  is  expelled  or  taken  in,  the  solution  in 
immediate  contact  with  the  membrane  is,  for  the  time  being,  concen- 
trated or  diluted  to  a  greater  extent  than  the  main  body  of  the  solution. 
The  final  adjustment  of  the  meniscus  can  not,  of  course,  be  reached 
until  the  whole  solution  has  become  homogeneous  through  diffusion 


THE  REGULATION  OF  TEMPERATURE.  55 

of  the  solvent.  Evidently  the  magnitude  of  the  so-called  extra  excur- 
sion will  depend  very  much  upon  the  rate  at  which  the  solvent  can 
pass  through  the  membrane  in  either  direction. 

The  magnitude,  and  therefore  the  importance,  and  the  peculiarities 
of  thermometer  effects  depend  upon  several  conditions  which  will  be 
briefly  recapitulated.  They  are: 

1.  The  relative  volumes  of  the  liquids  (solution  and  mercury)  and 
of  the  gas  in  the  cell.     Since  the  latter  is  always  very  small  as  compared 
with  the  former,  slight  disturbances  of  temperature  must  always  pro- 
duce large  thermometer  effects. 

2.  The  degree  of  the  "lag"  in  the  passage  of  solvent  through  the 
membrane,  which,  in  turn,  depends  upon  temperature  and  the  area 
and  age  of  the  membrane. 

3.  The  rapidity  with  which  the  changes  in  temperature  are  accom- 
plished.    The  relation  of  3  to  2  is  self-evident. 

4.  The  "lag"  in  the  distribution  of  solvent  through  the  solution  by 
diffusion,  which  produces  temporary  conditions  of  non-homogeneity 
in  respect  to  concentration. 

It  is  obvious  that,  in  one  sense,  we  could  have  no  thermometer  effects 
if  the  passage  of  solvent  through  the  membrane  and  its  subsequent 
uniform  distribution  by  diffusion  were  instantaneous,  since,  in  that 
case,  there  could  never  develop  in  the  cell  a  condition  of  over  or  under 
pressure.  In  other  words,  we  should  then  have  at  all  times  simply 
the  osmotic  pressure  of  a  solution  whose  concentration  varies  with  the 
temperature.  The  fact  that  diffusion  does  not  quite  keep  pace  with 
transferences  of  solvent  through  the  membrane  is  not  a  source  of 
serious  trouble,  but  in  the  lag  of  such  transferences  behind  fluctuations 
in  temperature  we  have  a  most  formidable  obstacle  in  the  way  of  the 
accurate  measurement  of  osmotic  pressure.  The  only  remedy  for  this 
unfortunate  situation  is  to  be  found  in  the  most  perfect  means  which 
can  be  devised  for  the  automatic  maintenance  of  constant  temperature. 

It  will  be  gathered  from  what  has  already  been  said  that  the  duration 
of  thermometer  effects  also  depends  principally  upon  the  rate  at  which 
the  solvent  is  able  to  diffuse  through  the  membrane.  In  practice  it 
is  found  that,  according  to  the  age  of  the  membranes,  they  may  last 
from  12  hours  to  4  days  after  the  bath  has  recovered  its  normal  tem- 
perature. As  regards  the  minimum  temperature  change  which  will 
give  a  sensible  thermometer  effect,  it  may  be  said  that  a  fluctuation 
of  0.01°  produces  a  movement  of  the  mercury  meniscus  which  can  be 
detected.  A  change  in  temperature  amounting  to  0.05°  gives  a  large 
thermometer  effect,  even  when  the  membrane  is  new.  It  has  not  been 
found  practicable  to  regulate  the  temperature  of  the  large  baths  which 
are  in  use  to  within  less  than  0.01°;  accordingly  the  meniscus  is  con- 
stantly moving  within  narrow  limits,  with  the  result  that  two  successive 
readings,  several  hours  apart,  are  rarely  quite  identical.  Fluctuations 


56  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 

of  0.02°  in  bath  temperature,  if  they  follow  one  another  with  regularity, 
are  tolerable,  because  the  thermometer  effects  due  to  rise  in  tempera- 
ture are  then  partially  neutralized  by  those  due  to  falling  temperature. 
Change  in  concentration  without  leakage. — It  has  been  seen  that  a 
solution  in  a  cell  may  become  permanently  concentrated  if  the  tem- 
perature rises  and  becomes  constant  at  a  higher  level;  also  that  it 
may  be  permanently  diluted  if  the  temperature  falls  and  becomes 
constant  at  a  lower  level.  There  are  also  two  cases  in  which  the  con- 
centration of  the  solution  may  be  altered  without  any  change  in  the 
temperature  of  the  bath.  Alterations  of  this  kind  occur  when  the 
cells  are  filled  with  solutions  whose  temperature  differs  from  that  of 
the  bath.  In  such  cases,  concentration  or  dilution  of  the  solution 
ensues,  according  as  the  temperature  of  the  solutions  is  lower  or  higher 
than  that  of  the  bath.  There  is,  however,  no  essential  difference 
between  the  two  modes  of  effecting  concentration  on  the  one  hand 
and  dilution  on  the  other,  since  both  depend  on  changes  in  volume  due 
to  changes  in  the  temperature  of  the  solutions. 

Except  for  the  maintenance  of  zero  temperature,  all  the  devices  for 
regulation  conform  to  one  principle,  which  may  be  stated  as  follows: 

//  all  the  water  or  air  in  a  bath  is  made  to  pass  rapidly  (1) 
over  a  continuously  cooled  surface  which  is  capable  of  reducing 
the  temperature  slightly  below  that  which  it  is  desired  to  main- 
tain, then  (2}  over  a  heated  surface  which  is  more  efficient  than 
the  cooled  one  but  which  is  under  the  control  of  a  thermostat, 
and  (3)  again  over  the  cooled  surface,  etc.,  it  should  be  practi- 
cable to  maintain  in  the  bath  any  temperature  for  which  the 
thermostat  is  set,  and  the  constancy  of  the  temperature  should 
depend  only  on  the  sensitiveness  of  the  thermostat  and  the  rate 
of  flow  of  the  water  or  air.  The  principle  is  a  general  one  and 
provides  for  the  maintenance  of  any  temperature  between  zero 
and  the  boiling-point  of  water.  Moreover,  any  desired  tempera- 
ture can  be  maintained  without  regard  to  the  temperature  of  the 
surrounding  atmosphere,  since  the  air  about  the  bath  must  always 
aid  in  the  work  either  of  the  cooling  or  the  heating  surface. 

The  "cooling"  surface  is  usually  furnished  by  a  series  of  brass  pipes 
through  which  water — under  a  constant  pressure — is  circulated.  If  the 
temperature  to  be  maintained  is  a  moderate  one,  i.  e.,  not  far  from  that 
of  the  atmosphere  but  above  that  of  the  hydrant  water,  the  latter  is 
passed  directly  through  the  circulating  system,  the  rate  of  flow  being 
so  regulated  as  to  maintain,  without  the  cooperation  of  the  heating 
surface,  a  temperature  which  is  slightly  too  low.  This  margin  between 
the  temperature  which  the  cooling  surface,  acting  alone,  will  maintain 
and  that  which  it  is  desired  to  keep  should,  for  economical  reasons,  be 


THE  REGULATION  OF  TEMPERATURE.  57 

made  as  small  as  is  consistent  with  safety.  If  the  temperature  to  be 
maintained  in  the  bath  is  very  near  to  or  below  that  of  the  hydrant 
water,  the  latter,  before  entering  the  circulating  system,  is  passed 
through  coils  of  metallic  pipes  which  are  surrounded  by  ice.  If  the 
desired  temperature  is  not  much  above  that  of  the  atmosphere,  the 
cooling  effect  of  the  surrounding  air  upon  the  exterior  of  the  bath  may 
suffice,  in  which  case  the  circulation  of  hydrant  water  is  discontinued. 
Finally,  if  the  temperature  to  be  maintained  is  considerably  above  that 
of  the  atmosphere,  the  system  within  is  connected  with  one  on  the  out- 
side, thus  forming  a  closed  circulating  system  which  is  partly  within 
and  partly  without  the  bath,  and  through  this  hot  water  is  circulated  by 
means  of  a  pump.  The  pump  may  be  situated  anywhere  in  the  system, 
i.  e.,  either  within  or  without  the  bath.  At  some  point  outside,  provision 
is  made  for  heating  the  water  by  gas  as  it  passes  through  the  system. 

Thus  far,  provision  for  the  cooling  surface  only  has  been  made.  Not- 
withstanding the  application  of  heat,  and  sometimes  a  good  deal  of  it, 
the  circulating  system  mentioned  above  is,  essentially,  a  cooling  device, 
inasmuch  as  its  purpose  is  to  reduce  the  temperature  of  the  bath  below 
that  which  is  to  be  maintained.  In  this  system,  for  economical  reasons, 
gas  is  used  for  heating  rather  than  electricity,  but  care  is  taken  so  to 
regulate  its  flow  that  a  "cooling  margin"  will  be  maintained,  whatever 
may  be  the  fluctuations  in  the  pressure  upon  the  gas.  Since  the  "cool- 
ing surface"  is  not  subject  to  exact  regulation,  it  must  never  be  allowed 
to  become,  in  effect,  a  "heating  surface, ' '  for  in  that  case  the  thermostat 
becomes  useless. 

The  "heating  surface"  usually  consists  of  one  or  more  copper  cyl- 
inders, in  which  are  inclosed  ordinary  electric  lamps,  which  serve  as 
stoves,  whose  purpose  is  to  overcome  the  "cooling  margin."  If  that 
margin  is  small — and  it  should  be  made  as  small  as  possible — the  con- 
sumption of  electricity  is  not  large.  Lamps  are  used  rather  than  other 
forms  of  electric  heating  devices,  because  one  can  always  select  among 
them  stoves  whose  capacity  is  suited  to  the  work  to  be  done,  and 
because  they  heat  up  and  cool  down  quickly,  which  is  an  important 
element  in  temperature  regulation. 

The  circulation  of  water  over  the  cooling  and  heating  surfaces  is 
effected  by  means  of  pumps,  and  it  will  be  seen  later  that  a  single 
pump  may  be  made  to  circulate  the  water  over  these  and  also  through 
the  cooling  system.  The  air  in  the  baths  is  circulated  by  means  of 
rotating  fans. 

THE  SCHEME  FOR  ELECTRICAL  REGULATION. 

The  device  by  means  of  which  the  margin  of  under-temperature  pro- 
duced by  the  so-called  "cooling  surface"  is  exactly  and  automatically 
overcome  is  shown  in  Figure  29.  Everything  not  essential  to  an  under- 
standing of  its  plan  is  omitted.  It  consists,  in  its  simplest  form,  of  (1) 


58 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


Relay  250  Ohms  lz      J 


Stove 


Line 


FIG.  29. — General  scheme  for  the  electric  regulation  of  bath  temperature. 
,  (Cz),  and  (€3)  condensers  spanning  spark-gaps  of  thermostat  and  relays;  (li),  (12),  and  (Is) 
lamps  spanning  the  same  spark-gaps  as  the  condensers;  (R)  the  master  relay  (250  ohms 
resistance) ,  which  is  operated  by  battery  and  may  control  any  number  of  stoves  in  parallel ; 
(Ri)  stove  relay  operated  by  battery  through  the  "local"  of  the  master  relay,  (a  with  arrows) 
course  of  current  through  battery,  thermostat  and  "main"  of  master  relay,  (b  with  arrows) 
course  of  current  through  battery,  "local"  of  master  relay  and  "main"  of  stove  relay. 
Current  of  main  line  passea  through  "local"  of  stove  relay  and  stove*  "Stops"  of  all  relays 
reversed. 


THE  REGULATION  OF  TEMPERATURE.  59 

a  battery  of  one  cell;  (2)  a  thermostat;  (3)  two  relays;  (4)  three  lamps 
(I)  and  three  condensers  (c)  which  span  the  spark  gaps  of  the  ther- 
mostat and  the  two  relays;  (5)  two  battery  circuits  which  operate  the 
relays  and  a  third  one  of  120  volts  which  passes  through  the  stove. 

1.  THE  BATTEBY. 

For  charging  purposes,  all  the  cells  in  use  (about  twenty  in  number) 
are  placed  in  series  upon  a  single  circuit  in  which  a  lamp  of  appro- 
priate resistance  is  also  inserted.  The  charging  is  continuous.  The 
cells  themselves,  though  in  series  on  a  single  circuit  for  charging,  are 
distributed,  in  numbers  corresponding  to  the  amount  of  work  to  be  done, 
at  points  conveniently  near  to  the  various  baths.  A  single  cell  suffices 
to  operate  the  system  at  any  point,  hence  each  cell  in  a  local  battery 
consisting  of  more  than  one  is  at  work  only  a  part  of  the  time,  i.  e., 
every  third  day,  if  the  local  battery  consists  of  three  cells. 

2.  THE  THEBMOSTAT. 

No  single  element  in  a  system  of  regulation  is  of  greater  importance 
than  the  thermostat.  Its  efficiency  depends  upon  a  considerable 
number  of  conditions,  some  of  which  are  worthy  of  more  than  a  passing 
mention.  That  the  mercury  must  be  of  exceptional  purity,  and  that 
its  volume  must  be  so  related  to  the  diameter  of  the  capillary  as  to 
secure  a  large  movement  of  the  meniscus  for  a  small  change  of  tem- 
perature, are  facts  too  obvious  to  require  discussion.  The  feature  to 
which  too  little  attention  is  usually  given  is  the  mechanism  for  adjust- 
ing the  contact  point.  This  should  be  located  directly  over  the  center, 
i.  e.,  the  highest  part  of  the  meniscus,  and  the  mechanism  should  be 
such  that  it  can  never  take  any  other  position  with  reference  to  the 
surface  of  the  meniscus.  The  best  form  of  thermostat  which  we  have 
in  use  is  shown  in  Figure  30.  The  platinum  rod  (a)  is  finished  to  a 
smooth  point  at  the  lower  end,  and  just  above  the  latter  is  a  guide 
(b)  of  glass,  which  is  designed  to  keep  the  point  near  the  center  of  the 
tube,  and  therefore  nearly  over  the  highest  part  of  the  meniscus.  At 
the  upper  end,  the  platinum  rod  (a)  is  firmly  set  in  the  threaded  brass 
rod  (c).  The  adjustment  is  made  by  means  of  the  nut  (e,  e),  which 
is  so  nicely  fitted  into  its  framework  that  it  can  move  in  a  horizontal- 
circular  direction  only.  The  dotted  circle  indicates  the  apertures 
through  which  the  adjusting  nut  (e,  e)  is  grasped  between  the  thumb 
and  forefinger  when  the  contact-point  is  to  be  lowered  or  raised.  The 
guide  (6),  which  must  fit  the  tube  rather  loosely,  does  not  suffice  to 
compel  the  contact-point  to  keep  exactly  its  proper  position  with 
reference  to  the  meniscus.  The  rod  (a)  is  never  absolutely  straight, 
hence  the  point,  if  the  rod  is  allowed  to  turn,  will  describe  a  circle 
over  the  meniscus.  For  this  reason,  having  once  correctly  adjusted 
the  point,  its  motion  must  be  limited  to  the  vertical  direction;  in  other 
words,  the  threaded  rod  (c)  must  not  be  allowed  to  turn  with  the  nut 


60 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


(e,  e).  The  device  by  which  any  movement  of  (c)  in  a  horizontal  direc- 
tion is  prevented  is  seen  in  the  upper  part  of  the  figure.  The  two  nuts — 
(/)  and  (g) — are  threaded  internally  to  fit  c. 
The  lower  and  wider  one  is  bored  at  two  oppo- 
site points  for  two  rods,  of  which  only  one  (h)  is 
seen  in  the  figure.  Corresponding  to  the  holes 
in  g,  two  other  holes  are  bored  in  the  brass  cap  (i) . 
Having  found  the  correct  position  for  the  con- 
tact-point, the  rods  h  and  hi  (not  seen),  are  in- 
serted, and  the  set  nut  (/)  is  turned  down  upon 
g.  The  rod  (c),  though  still  free  to  move  in  a 
vertical  direction,  can  not  now  turn  with  the 
adjusting  nut  (e,  e). 

Sparking  at  the  point  of  contact  in  the  ther- 
mostat is  effectively  prevented  by  spanning  the 
spark  gap  (Figure  29)  with  a  lamp  of  high  volt- 
age (h)  and  a  condenser  (d).  Since  removal 
of  the  condenser  has  not  been  found  to  induce 
visible  sparking  at  the  point  of  contact,  it  is 
doubted  whether  it  serves  any  useful  purpose. 
As  a  matter  of  fact,  it  is  often  omitted.  The 
lamp  (li)  which  is  ordinarily  employed  is  one 
of  16  candle-power  at  250  volts. 

The  water  in  the  baths  is  always  in  rapid 
motion,  and  a  thermostat  which  is  immersed 
in  it  without  protection  is  subject  to  slight  but 
constant  jarring,  which  results  hi  a  phenom- 
enon which  has  come  to  be  known  under  the 
name  of  "frosting."  The  air  between  the  mer- 
cury in  the  thermostat  and  the  glass  wall  col- 
lects at  a  multitude  of  points  in  minute  bubbles, 
which  give  to  the  glass  a  frosted  appearance. 
In  the  course  of  time,  the  bubbles  of  gas  coalesce, 
forming  aggregations  so  large  that  the  true  FIG.  so.— The  thermostat. 
nature  of  the  phenomenon  can  be  discovered  by  (a)  Platinum  rod,  pomtedTat 
the  naked  eye.  The  first  indication  which  one  }ower  endl  <6>  elass  guide  to 

,,  ;          •*     *  *<>       .*      «ii  -i        keep   (a)  in  center  of  tube; 

usually  receives  that  'frosting    has  commenced      (C)  threaded  brass  rod ;  (e)  in- 
is  a  "chattering"  of  the  relays. 

"Frosting"  can  be  prevented  by  protecting 
the  thermostats  from  the  shock  of  the  moving 
water  by  surrounding  them  with  metallic  tubes, 
or  by  exhausting  them  before  introducing  the 
mercury.  The  boiling-out  process  employed 
for  barometers  has  not  been  found  practicable  for  thermostats  of  the 
form  used  by  us. 


closed  nut  for  adjusting  con- 
tact point;  (/),  (g),  and  (h) 
arrangements  for  preventing 
all  movements  of  the  contact 
point,  except  in  a  vertical  di- 
rection. The  figure  to  the 
left  shows  the  glass  parts  of 
the  thermostat. 


THE  REGULATION  OF  TEMPERATURE.  61 

3.  THE  MASTER  RELAY. 

The  master  relay  (R,  Figure  29),  so  called  because  it  controls  all 
the  relays  connected  directly  with  the  stoves,  has  a  resistance  of  250 
ohms,  and  is  of  the  type  commonly  used  in  telegraph  lines.  High 
resistance  in  this  relay  is  desirable  in  order  to  reduce  to  a  minimum 
the  current  which  must  pass  through  the  thermostat.  The  course  of 
the  only  circuit  which  passes  through  the  thermostat  and  the  magnets 
of  the  master  relay  is  indicated  in  Figure  29  by  the  letter  a.  The 
current  in  this  circuit  amounts  to  8  milamperes  plus,  of  course,  what 
may  pass  through  the  high-resistance  lamp  (7i). 

4.  THE  MINOK  RELAY. 

The  "local "  of  the  master  relay  is  made  the  " line  "  circuit  of  the  minor 
relay  (Ri).  The  course  of  this  circuit  is  indicated  by  the  letter  b.  The 
spark-gap  of  the  master  relay,  like  that  of  the  thermostat,  is  spanned  by 
a  lamp  (k)  and  a  condenser  (c2),  although  the  latter  is  often  dispensed 
with.  The  minor  relay  has  a  resistance  of  only  20  ohms.  Its  spark-gap 
is  also  spanned  by  a  lamp  (Z3)  and  a  condenser  (c3).  The  stove  circuit, 
as  shown  in  the  figure,  passes  through  the  " local"  of  the  minor  relay. 

In  all  of  the  relays — both  master  and  minor — the  usual  arrangement 
of  the  "stops"  is  reversed,  so  that  the  closing  of  the  circuit  through 
the  thermostat  opens  the  circuit  through  the  " locals"  of  the  relays. 
Obviously,  what  it  really  does  is  to  cut  down  the  current  in  these 
circuits  by  throwing  into  them  the  resistance  of  the  lamps  (72  and  13). 
None  of  the  three  circuits  employed  in  the  system  is  ever  fully  broken. 
But  the  currents  which  pass  continuously  through  the  lamps  (li  and  Z2) 
are  insufficient  to  operate  the  relays,  while  that  which  passes  continu- 
ously through  13  does  not  overheat  the  bath. 

By  putting  the  minor  relays  in  parallel,  a  single  master  relay  is  made 
to  operate  any  number  of  stoves.  In  some  of  our  baths,  the  "master" 
controls  as  many  as  eight  stoves. 

For  convenience  in  use,  the  system  of  electrical  control  is  divided 
into  two  units,  and  the  apparatus  belonging  to  each  is  permanently 
installed  on  a  portable  board  which  may  be  fixed  in  any  suitable 
position  with  reference  to  a  bath.  All  connections,  except  the  perma- 
nent ones  on  the  boards,  are  made  by  means  of  flexible  leads,  to  the 
ends  of  which  are  attached  insertion  plugs.  On  the  "master  board" 
are  placed  and  wired  together  the  master  relay,  the  lamp  which  spans 
the  spark-gap  of  its  "local,"  and  plug  attachments  for  the  battery, 
the  condenser,  and  for  several  stove  boards.  On  each  of  the  minor 
or  stove  boards  are  placed  and  wired  together  two  minor  relays  in 
parallel  for  as  many  stoves,  the  lamps  which  span  their  spark-gaps, 
and  plug  attachments  for  the  master  board,  the  condensers,  and  the 
stoves.  The  "unit"  boards  of  each  kind  are  uniform  in  arrangement 
and  size  and  are  therefore  interchangeable. 


62 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


THE  BATH  FOR  0°. 

As  previously  intimated,  the  bath  which  is 
tions  at  0°  does  not  conform  to  the  general 
principle  upon  which  all  baths  for  higher 
temperatures  are  constructed.  The  essential 
difference  between  it  and  the  others  is  that 
in  the  bath  for  0°  there  is  no  forced  circula- 
tion of  water  and  air.  An  attempt  was  made 
to  construct  a  bath  in  partial  conformity 
with  the  general  principle  in  question.  In 
this,  a  large  mass  of  ice  was  made  the  "cooling 
surface,"  and  the  exterior  of  the  bath  the 
"heating  surf  ace."  Water  was  passed  rapidly 
over  the  ice,  and  then  over  the  compart- 
ments in  which  the  cells  were  located.  It  was 
found,  however,  that  the  lowest  temperature 
which  it  was  practicable  to  maintain  in  this 
manner  was  always  a  little  above  0° ;  and  that 
owing  to  imperfect  control  of  the  heating 
surface,  the  temperature  was  subject  to  con- 
siderable fluctuations  which  produced  large 
thermometer  effects. 

The  bath  which  was  finally  evolved  for  the 
determination  of  osmotic  pressure  at  zero  is 
seen  in  Figures  31, 31 L,  and  31 M.  The  ap- 
paratus, which  is  made  of  heavy  galvanized 
sheet  iron,  consists  of  three  principal  parts: 
First,  a  can  (A),  in  which  the  cells  are  placed ; 
second,  a  much  larger  one  (£),  in  which  A 
is  suspended  by  means  of  the  arrangement 
seen  in  Figure  31  L;  and  third,  the  cylinder 
(C),  which  shuts  down  tightly  upon  B. 
There  is  an  inclosed  chamber  (e)  running 
through  the  whole  length  of  C  and  open  at 
both  ends,  in  which  are  located  the  upper 
ends  of  the  manometers  and  the  two  ther- 
mometers which  are  seen  in  the  figure.  The 
thermometers  and  manometers  are  exposed 
to  view,  when  a  reading  is  to  be  made,  by 
opening  the  felt-lined  door  (/).  In  order 
that  the  door  may  be  opened  and  closed 
from  the  outside,  the  detachable  rod  (g)  is 
made  to  pass  through  the  top  of  the  larger 
bath  (to  be  described  later)  which  surrounds 
A,  B,  and  C.  The  bottoms  of  both  A  and 
no  water  can  collect  in  the  cans. 


employed  for  determina- 


FIG.  31. — Interior  ice  bath  for  meas- 
uring osmotic  pressure  at  0°. 

(A)  Galvanized-iron  can  contain- 
ing cells;  (B)  galvanized-iron 
ice-container  surrounding  (A) ; 
(C)  galvanized-iron  ice-container 
which  shuts  down  upon  (B) ;  (e) 
protected  compartment  for  ma- 
nometers and  thermometers;  (/) 
padded  door  to  (e)  •  (g)  arrange- 
ment for  opening  and  closing  the 
door  from  above  the  outer  ice  bath, 
which  is  not  shown  in  the  figure; 
(L)  arrangement  for  suspending 
(A)  in  (B) ;  (M)  cover  to  (4). 

B  are  perforated  so  that 


THE  REGULATION  OF  TEMPERATURE.  63 

The  "larger  bath"  referred  to  above  is  one  of  those  ordinarily  used  for 
measurements  of  pressure  at  higher  temperatures.  To  prepare  it  for 
use  at  0°,  it  is  stripped  of  all  its  interior  accessories,  including  circulating 
pipes  and  pump,  leaving  the  copper-lined  rectangular  tank  entirely 
empty.  On  the  bottom  of  this,  in  the  center,  is  placed  a  staging  about 
5  centimeters  high,  on  which  rests  the  ice-filled  arrangement  consisting 
of  A,  B,  and  C.  All  the  space  in  the  tank  which  is  not  occupied  by  A, 
B,  and  C  is  filled  with  closely  packed  broken  ice,  and  the  water  which 
collects  upon  the  bottom  is  removed  by  means  of  an  automatic  siphon. 
All  the  space  in  the  upper  part  of  the  bath — usually  designated  as  the 
"air  space" — which  is  not  occupied  by  the  upper  part  of  C  is  filled  with 
ice  containers  of  such  form  that  they  surround  C  except  directly  in 
front  of  the  door  (/).  One  of  these  occupies  the  space  between  the 
upper  end  of  C  and  the  top  of  the  outer  bath,  the  upper  end  of  the  cham- 
ber (e)  being  covered  to  prevent  the  entrance  of  water. 

All  of  the  ice-containers  in  the  air  space  above  are  open  at  the  lower 
end,  so  that  the  broken  ice  moves  constantly  downwards  as  it  melts 
away  underneath,  keeping  the  tank  below  and  also  the  can  (B)  always 
full.  A  little  over  150  kilograms  of  ice  are  required  to  fill  the  bath 
properly,  and  the  amount  of  fresh  ice  which  it  is  necessary  to  introduce 
daily  is  between  25  and  30  kilograms. 

The  container  above  C  and  C  itself,  after  the  ice  in  them  has  been 
picked  out,  can  be  lifted  through  the  opened  top  of  the  outer  bath  when- 
ever cells  are  to  be  removed  from  A.  If  cells  are  to  be  introduced,  all 
parts  of  the  bath  except  C  and  the  container  above  it  are  closely  packed 
with  ice,  and,  after  waiting  until  the  temperature  in  A  has  fallen  to  0°, 
the  cells  are  placed  in  position.  C  is  brought  down  upon  B  and  packed 
with  ice.  Finally,  the  container  which  belongs  directly  above  C  is 
placed  in  position  and  filled  with  ice. 

The  arrangement  described  above  serves  its  purpose  perfectly.  The 
temperature  in  A  does  not  deviate  sensibly  from  0°.  There  are,  there- 
fore, no  appreciable  thermometer  effects.  The  temperature  of  the 
manometer  space  (e)  may  be  affected  somewhat  by  the  lamp  used  in 
reading,  unless  one  interposes  a  screen  for  the  purpose  of  cutting  down 
the  heating  effect  of  the  light.  We  have  employed  for  this  purpose  a  4 
per  cent  solution  of  nickel  sulphate,  which — as  determined  by  means  of  a 
thermocouple — reduces  the  heating  effect  of  the  lamp  nearly  99  per  cent. 

BATHS  FOR  MAINTENANCE  OF  TEMPERATURE  ABOVE  ZERO. 

Descriptions  of  the  earlier  forms  will  be  omitted.  The  baths  which 
were  first  employed  in  an  attempt  to  measure  osmotic  pressure  were 
found  to  be  incapable  of  maintaining  sufficiently  exact  temperatures. 
In  other  words,  the  thermometer  effects  produced  by  their  fluctuations 
of  temperature  were  intolerably  large.  The  baths  which  are  now  used 
and  which  will  be  described  are  the  products  of  a  persistent  attempt  to 
reduce  these  effects  to  harmless  proportions.  They  all  belong  to  cer- 


64 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


•To  relay 
FIG.  32. — 60-liter  galvanized-iron  bath  for  intermittent  use. 

(a)  and  (b)  wooden  base;  (c)  hair  padding;  (d)  water-tight  inclosure  for  electric  stove  (lamp); 
(e)  electric  stove;  (/)  and  (g)  removable  base  for  lamp;  (h)  frame  for  lamp;  (i)  supports  for 
circulating  system;  (f)  coil  of  block-tin  pipe  for  the  circulation  of  cold  or  hot  water  (shown 
better  in  Figure  32,  B) ;  (k)  and  (f)  pipes  by  which  water  enters  and  leaves  the  circulating 
system  (f) ;  (m)  and  (ri)  cylinder  open  at  top  and  resting  on  pegs  between  coils  of  (f) ;  (o)  holes 
for  escape  of  gas  expelled  from  water;  (p)  holes  for  escape  of  water  into  outer  bath;  (r)  pro- 
peller for  pumping  water  over  heated  chamber  (d);  (s)  pulley;  (<)  oil  cup;  (M)  thermostat; 
(»)  thermometer;  (w)  adjustable  iron  frame  for  fixing  the  propeller  in  place;  (x)  vertical  pipe 
leading  to  pressure  regulator. 

A.  Adjustable  support  for  bottles,  etc.        B.  Block-tin  circulating  system  (j  in  Figure  32). 

C.  Constant-pressure  arrangement,  (ad)  Stand-pipe;  (bd)  overflow;  (cd)  stopcock  for  the  regu- 
lation of  flow  of  water  through  circulating  system  in  bath;  (dd)  entrance  place  of  water-supply. 


THE  REGULATION  OF  TEMPERATURE.  65 

tain  fairly  distinct  types,  though  differing  much  among  themselves  in 
respect  to  details.     An  example  of  each  type  will  be  given. 

TYPE  I. 

This  bath  (Figures  32  and  33),  which  was  designed  for  general  but 
intermittent  use  in  connection  with  the  work,  consists  of  a  cylindrical 
tank  of  galvanized  iron,  which  holds  from  60  to  80  liters.  It  is  sur- 
rounded by  a  thick  covering  of  hair  felt  (c,  Figure  32),  and  rests  upon  a 
wooden  base  (a,  a),  which  is  raised  above  the  table  or  floor  by  the  blocks 
and  rubber  pieces  (6,  6).  Inverted  over  the  hole  in  the  center,  and 
riveted  and  soldered  to  the  bottom  of  the  bath,  is  the  cylinder  (d,  d,  d), 
which  serves  as  a  receptacle  for  the  lamp  (e),  or  any  other  suitable  kind 
of  electrical  heating  device.  The  lamp  is  mounted,  in  the  manner 
indicated  in  the  figure,  upon  the  removable  block  (/),  which  is  held  in  its 
place  by  buttons  screwed  to  the  base  (a,  a).  Resting  upon  the  frame- 
work i,  i  (Figure  32)  and  i,  i,  i  (Figure  32  B)  is  the  continuous  block-tin 
pipe  (j,  j,  j,  j),  through  which  the  hydrant  water  circulates.  The  cyl- 
inder (d,  d,  d)  constitutes  the  "heating"  surface,  and  the  pipe  (j,  j,  j,  j) 
the  "cooling"  surface.  The  running  water  enters  the  bath  at  k  and 
leaves  at  I  (Figures  32  and  32  B).  The  course  of  the  water  in  the  pipe, 
after  entering  the  bath,  is  continuously  horizontal  or  upward — never 
downward.  This  arrangement  is  necessary  in  order  to  prevent  the 
lodgment  of  air  in  any  part  of  the  pipe.  The  successive  coils  of  pipe 
(six  in  number)  are  separated  by  the  pegs  seen  in  Figures  32  and  32  B, 
and  on  these  rests  the  galvanized  iron  disk  (m,  m).  The  hood  (n,  ri),  of 
the  same  material,  shuts  down  tightly  over  a  flange  on  the  disk  (m,  m) 
and  is  adjusted  and  secured  in  its  place  by  set  screws  directed  towards 
d,  d.  The  form  of  the  hood  will  be  clear  from  the  figure,  and  it  is  neces- 
sary only  to  call  attention  to  the  small  holes  for  the  escape  of  air  at  o,  o, 
and  to  the  larger  holes  at  p,  p,  through  which  much  of  the  water  raised 
by  the  propeller  (r)  escapes  into  the  outer  bath. 

The  purpose  of  the  various  parts  which  go  with  the  propeller — the 
adjustable  cross-bar  (w,  w),  which  is  clamped  to  the  sides  of  the  bath, 
the  oil  cup  (0,  the  pulley  (s),  etc. — is  sufficiently  obvious. 

It  is  quite  essential  that  the  hydrant  water  which  flows  through  the 
pipe  shall  be  under  constant  pressure,  otherwise  much  water  and  heat 
are  necessarily  wasted.  The  arrangement  by  which  the  constant  pres- 
sure is  secured  is  shown  in  Figure  32  C.  It  consists  of  a  large  standpipe 
(ad)  with  an  overflow  (bd)  near  the  top.  The  water  from  the  tap  enters 
at  the  bottom  (dd)  and  passes  to  the  bath  through  cd,  where  the  flow  is 
controlled  by  a  stopcock.  The  circulating  water  is  thus  brought  under 
an  invariable  pressure,  and  it  is  possible  to  regulate  the  quantity  pass- 
ing through  the  bath  with  considerable  nicety  and  for  any  length  of 
time.  At  the  highest  point  in  the  waste  pipe  (z,  Figure  32)  is  placed  a 
vent  through  which  any  air  carried  along  by  the  water  may  escape. 


66  OSMOTIC   PRESSURE    OF   AQUEOUS    SOLUTIONS. 

If  the  temperature  of  the  hydrant  water  is  above  that  at  which  the 
bath  is  to  be  maintained,  the  block-tin  spiral  pipe  (Figure  33)  is 
inserted  between  the  tap  and  dd  (Figure  32  C).  To  cool  the  water 
which  enters  at  a,  before  it  passes  through  6,  dd,  and  cd  into  the  bath, 
the  large  and  well-protected  box  in  which  the  spiral  is  located  is  packed 
with  ice.  In  this  manner,  it  is  practicable  to  maintain  a  quite  low 
temperature  in  the  warmest  weather. 

The  bath  just  described  is  used  principally  for  bringing  solutions  to 
temperature  and  for  maintaining  them  at  temperature,  for  the  com- 
parison of  thermometers  and  the  adjustment  of  thermostats,  and  for 
other  similar  purposes.  The  various  instruments  and  vessels  are  held 
in  their  places  in  the  bath  by  means  of  adjustable 
supports  or  clamps,  of  which  that  for  bottles  is  o  [Q 

shown  in  Figure  32  A  .  (—  -^_J 

The  maintenance  of  any  temperature  from  a      (^_  ;  P^) 

little  above  0°  to  that  of  the  room  can  be  readily 
accomplished  by  means  of  the  hydrant  water,  with 
or  without  ice.  If,  however,  a  temperature  above 
that  of  the  room  is  to  be  maintained,  the  flow  of  the 
hydrant  water  is  cut  off.  The  outer  surface  of  the 
bath  and  the  exposed  surface  of  the  water  then 
become  the  "cooling"  surface,  and  the  bath  works 
on  precisely  the  same  principle  as  before.  If  a 


temperature  above  50°  is  to  be  maintained,  the    FIG.  33.—  Coil  of  block- 
consumption  of  electric  energy  becomes  expensive      jJiaJJ  watT'hefore 
in  large  baths,  and  it  is  well  to  accomplish  a  portion       it  enters  the  circuiat- 
of  the  heating  by  means  of  gas.     This  is  done  in      ^ysiem  within  the 
various  ways,  but  most  simply  by  removing  the     (a)  Eatrance.  (6)exit. 
wooden  base  and  mounting  the  bath  on  a  large 
iron  tripod  over  a  ring  burner  of  suitable  diameter,  taking  care,  of 
course,  so  to  regulate  the  quantity  of  burning  gas  that  the  stove  alone 
can  not  raise  the  temperature  of  the  bath  to  the  required  height. 

TYPE  II. 

Figure  34  represents  one  of  the  more  recent  forms  of  bath,  in  which 
the  membranes  are  deposited  and  in  which  the  cells  and  the  solutions 
are  maintained  at  the  temperature  at  which  osmotic  pressure  is  to  be 
measured. 

The  "cooling"  surface  is  furnished  by  the  horizontal  brass  pipes 
(1  to  8)  .  The  hydrant  water,  cooled  by  ice  if  necessary,  enters  by  pipe  1 
and,  after  circulating  through  all  the  six  intervening  pipes  in  the  order 
in  which  they  are  numbered,  it  leaves  the  bath  by  pipe  8.  For  all  tem- 
peratures below  the  highest  temperature  of  the  room,  it  is  necessary  to 
keep  some  water  in  circulation  in  this  system  of  pipes.  The  amount  to 
be  sent  through  will,  of  course,  depend  on  the  difference  between  the 


THE  REGULATION  OF  TEMPERATURE. 


67 


temperature  of  the  hydrant  water  and  that  which  is  to  be  maintained 
in  the  bath.  If  the  hydrant  water  is  to  be  cooled  before  entering  the 
bath,  as  when  a  low  temperature,  e.  g.,  5°,  is  to  be  maintained  in  sum- 
mer, it  is  first  passed  through  the  coils  of  pipe  seen  in  Figure  33,  which 
are  embedded  in  ice.  The  arrangement  shown  in  Figure  32  C  is  also 
employed  in  this  bath  to  secure  a  constant  pressure  upon  the  circulating 
water. 


Fia.  34. — Rectangular  bath  for  general  laboratory  use. 

(1)  to  (8)  Brass  tubes  for  circulation  of  hydrant  water;  (9)  and  (10)  copper  cylinders,  opening  on 
opposite  sides  of  the  bath,  for  the  lamps;  (11)  pump;  (12)  and  (13)  pipes  through  which  water 
is  drawn  out  of  the  bath  and  over  the  gas  stoves  seen  at  the  end;  (14)  large  pipe  through 
which  water  heated  by  the  gas  stoves  is  drawn  and  delivered  at  (11). 

A  word  of  caution  may  be  given  regarding  the  valves  to  be  used  when 
a  constant  pressure  on  running  water  is  to  be  maintained.  Our  first 
pressure  arrangements  were  constructed  in  accordance  with  correct  prin- 
ciples, so  far  as  we  knew,  but  it  was  found  that  they  would  not  maintain 
constant  pressures.  The  flow  of  water  diminished  continually,  and 
very  small  streams  ceased  altogether  after  a  time.  After  a  long  search, 
the  difficulty  was  located  in  the  valves.  Those  we  were  using — the 
so-called  "gate-valves" — were  found  to  be  so  constructed  as  to  permit 
the  accumulation  of  the  gas  which  is  expelled  from  water,  when  its  tem- 
perature is  raised,  to  such  an  extent  as  to  impede  the  flow  of  the  water, 
and  to  stop  it  altogether  if  only  a  little  were  passing  through  the  valves. 
After  replacing  the  "gate-valves"  by  others  of  the  common  lever 
variety,  the  difficulty  disappeared. 

The  "heating"  surface  is  furnished  by  the  two  copper  cylinders  (9  and 
10),  the  latter  of  which  is  broken  in  order  to  show  the  location  of  the 
stoves.  The  large  wooden  box  is  lined  with  copper,  and  the  two  copper 
cylinders  in  question  extend  entirely  through  it  from  side  to  side,  and  are 
opened  at  both  ends  upon  the  outside  of  the  bath.  They  are  closed  with 
caps,  upon  the  inside  of  which  are  fastened  the  lamps.  Provision  is 
thus  made  for  four  lamps  which  are  usually  of  16  candle-power,  though 
lamps  of  8  candle-power  often  suffice  at  low  temperatures.  The  lamps 
(stoves)  are  regulated  according  to  the  scheme  already  explained. 


68       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

The  circulation  of  the  water  in  the  bath  over  the  cooling  and  heating 
surfaces  is  effected  by  means  of  the  pump  (11).  It  enters  the  pipes 
(12  and  13),  which  end  just  inside  the  rear  end  of  the  bath,  and  passes, 
in  the  direction  of  the  arrows,  into  the  large  pipe  (14),  thence  to  the 
pump  and  out  again  into  the  open  bath.  It  will  be  observed  that  the 
tendency  is  to  draw  the  colder  water  upon  the  bottom  of  the  bath  very 
rapidly  into  the  pipes  (12  and  13),  but  that,  as  it  enters  these,  it  is  neces- 
sarily mixed  with  water  which  has  passed  over  the  heating  surfaces 
(9  and  10).  Many  positions  for  the  heating  surfaces  have  been  tried, 
but  that  given  in  Figure  34  has  been  found  most  satisfactory. 

The  rate  of  pumping  depends  upon  what  is  found  to  be  necessary  in 
order  to  secure  identical  temperatures  at  the  two  ends  and  the  middle  of 
the  bath.  Ample  provision  is  made  for  any  rate  which  may  be  required. 
A  moderate  rate  for  some  of  the  larger  baths  is  400  liters  per  minute. 

The  purpose  of  extending  the  pipes  (12,  13,  and  14)  outside  of  the 
bath,  where,  at  their  junction,  the  circulating  water  passes  over  a  gas 
stove,  is  obviously  to  economize  electricity.  The  rule  here,  as  in  all 
other  baths,  is  to  utilize  gas  for  heating  purposes  to  the  utmost  safe 
limit,  leaving  for  the  electrical  appliances  only  so  much  as  is  indispen- 
sable for  regulation. 

Five  baths  of  Type  II  are  in  use,  varying  in  size  and  equipment,  but 
all  conforming  in  principle  to  that  just  described.  Plate  3  presents 
their  appearance,  also  that  of  the  baths  of  Type  I. 

TYPE  III. 

An  example  of  one  kind  of  bath  in  which  osmotic  pressure  is  measured 
is  shown  in  Figures  35  and  36.  The  first  (Figure  35)  represents  the 
lower  part,  which  is  filled  with  water  and  in  which  are  located  circulat- 
ing systems  similar  to  those  described  under  Type  II.  In  the  second 
(Figure  36)  is  seen  the  upper  part  of  the  bath,  the  so-called  "air 
space."  Both  divisions  are  lined  with  copper  and  are  separated  by  a 
vapor-tight  brass  plate  (1,  Figure  35  or  36),  which  is  divided  diagonally 
across  the  bath  into  two  parts  which  are  reunited  by  the  brass  strip  (2) . 
The  brass  plate  (1)  is  screwed  down  upon  the  upper  edge  of  the  outer 
wooden  bath,  but  between  the  two,  as  also  between  2  and  1,  strips  of 
sheet  rubber  are  placed  to  prevent  the  passage  of  water  vapor  from  the 
lower  part  of  the  bath  into  the  "air  space"  above.  The  reason  for  keep- 
ing the  latter  as  dry  as  possible  will  appear  later.  Six  lead-weighted 
copper  cans  are  suspended  from  the  brass  covering  plate,  the  flange  of 
each  resting  upon  a  rubber  collar;  they  serve  as  receptacles  for  the 
cells.  During  a  measurement  of  pressure,  the  space  in  the  cans  above 
and  around  the  cells  is  filled  with  wool.  In  two  of  the  three  baths  of 
Type  III,  the  cans  have  been  replaced  by  two  long,  narrow  troughs, 
whose  depth  is  equal  to  that  of  the  cans.  The  troughs  have  covers 
which  are  divided  into  many  readily  removed  sections.  A  bath  so 
arranged  will  easily  accommodate  24  cells  instead  of  6. 


MORSE 


THE  REGULATION  OF  TEMPERATURE. 


69 


Fio.  35. — Lower  half  of  rectangular  bath  for  measuring  osmotic  pressure — -the  "water  compartment." 
(1)  and  (2)  Brass  vapor-tight  cover  from  which  are  suspended  the  copper  cans  which  contain  the  cells;  (3), 
(7),  and  (8)  one-half  of  the  brass  tubes  belonging  to  the  circulating  system  for  hydrant  water;  (9)  and 
(10)  copper  tubes  which  open  at  both  ends  on  the  outside  of  the  bath;  (12)  and  (13)  tubes  through  which 
the  water  is  drawn  from  bath  and  over  the  gas  stoves;  (14)  large  pipe  through  which  water  heated  by  the 
gas  stoves  is  again  pumped  into  the  bath. 


Fio.  36. — Upper  half  of  rectangular  bath  for  measurement  of  osmotic  pressure — "air"  or 

"manometer  compartment." 

(1)  and  (2)  Vapor-tight  cover  to  "water  compartment;"  (3),  (4),  (5),  and  (6)  screened  lamps;  (7)  brass 
pipes  for  circulation  of  hydrant  water;  (8)  circulating  system  for  hot  water;  (9)  electric  fan;  (10) 
thermostat. 


70 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


It  will  be  seen  (Figure  35)  that  the  ar- 
rangements in  the  lower  part  of  the  bath  are 
nearly  identical  with  those  of  the  bath  de- 
scribed under  Type  II.     There  is  the  same 
system  of  brass  pipes  (3,  7,  8,  etc.)  for  the 
circulation  of  hydrant  water,  and  the  same 
arrangement  for  pumping  water  out  of  the 
bath  (through  12  and  13)  to  be  heated  by  a 
gas  stove  and  returned  through  the  large 
pipe  (14) .   There  is  also 
in  both  baths  the  same 
pro  vision  for  the  "heat- 
ing   surface,"     except 
that  the  copper  cylin- 
ders (9  and  10),  in  which 
the  lamps  are  located,  are 
somewhat  differently  placed 
in  the  two  cases. 

The  copper-lined  upper 
part  of  the  bath — the  "air 
space" — (Figure  36)  is  elec- 
trically heated  by  means  of 
the  lamps  (3,  4,  5,  and  6), 
which  are  shaded  for  the  pro- 
tection of  the  various  instru- 
ments containing  mercury. 
There  are  two  systems  of 
pipes  in  the  air  space.  That 
seen  in  the  top  (7)  is  for  the 
circulation  of  hydrant  water. 
It  serves  the  same  purpose 
in  the  air  space  as  the  system 
of  pipes  (3,  7,  8,  etc.)  in  the 
lower  part  of  the  bath .  The 
system  of  pipes  situated  at 
the  end  of  the  bath  (8)  is  for 

,,         .         ,,.  .  ,      ,          ,         (1),  (2),  and  (3)  Gas  lamps  (outside  of  bath)  for  heating 

tne  Circulation  OI  not  Water.      circulating  water;  (4)  pump;  (5)  and  (6)  stopcocks  which 
It  maV  also  be  USed  for  Cold      are  use^  when  hydrant  water  is  to  be  circulated  through 
rm         .     .      ,  v  the  pipes. 

water.    The  air  in  the  upper 

part  of  the  bath  is  kept  in  circulation  by  means  of  the  fan  (9). 

The  heating  and  pumping  arrangements  for  the  hot  water  are  situated 
on  the  outside  of  the  bath.  Their  relation  to  what  is  seen  on  the  inside 
is  shown  in  Figure  37.  The  gas  burners  (1,  2,  and  3)  heat  the  water  on 
its  way  to  the  pump  (4),  from  which  it  is  returned  in  the  direction  of  the 
arrows.  When  the  system  is  used  for  the  circulation  of  hydrant  water, 
the  water  enters  through  5  and  leaves  through  6.  In  one  of  the  baths 


FIG.  37. — Hot-water  circulating  system  with  end  of 
bath  removed. 


MORSE 


PLATE  4 


Rectangular  bath,  end  view. 


THE  REGULATION  OF  TEMPERATURE.  71 

of  Type  III,  the  interior  portion  of  the  system  has  been  replaced  by  a 
single  large  pipe  of  ring  form,  which  is  so  arranged  on  the  inside  that  the 
hot  water  is  returned  to  the  upper  part,  while  the  colder  water  is  con- 
stantly pumped  out  of  the  bottom. 

The  motor  fan  (9)  is  employed  to  keep  the  air  in  the  inclosed  space  in 
circulation  over  the  heated  pipes  and  over  the  lamps;  but  it  serves  also 
to  keep  the  manometers  gently  but  constantly  agitated,  and  thus  to 
overcome  the  tendency  of  the  mercury  to  lag  in  the  tubes.  This  agita- 
tion is  increased  to  any  desired  extent  by  attaching  bits  of  stiff  paper 
to  the  upper  ends  of  the  manometers. 

The  external  appearance  of  the  bath  is  seen  in  Plates  4  and  5. 
In  the  latter,  the  system  of  pipes  for  the  circulation  of  hydrant  water, 
which  should  be  seen  at  the  top  of  the  interior,  has  been  removed,  as 
this  bath  is  but  little  used  for  temperatures  below  that  of  the  air. 

The  other  baths  of  the  same  general  type  (two  in  number)  were 
planned  with  reference  to  the  measurement  of  osmotic  pressure  at  low 
or  very  moderate  temperatures.  They  differ  from  the  bath  described 
mainly  in  the  care  which  has  been  taken  to  protect  the  interior  from 
external  temperature  conditions.  Their  wooden  walls  are  all  double 
and  the  intervening  space  is  filled  with  hair.  Moreover,  the  small 
rooms  in  which  they  are  located  are  made  subject  to  temperature  regu- 
lation by  means  of  pipes  covering  the  ceiling  through  which  hydrant 
water  is  circulated  when  necessary.  A  further  means  of  cooling  these 
bath  rooms  consists  of  a  chute  opening  upon  the  outside  of  the  building, 
through  which  air  is  introduced  into  the  room  at  any  desired  rate  by 
means  of  a  rotary  fan.  Formerly  it  was  attempted — by  means  of  a  cir- 
culating system  for  hydrant  water,  by  the  introduction  of  a  regulated 
quantity  of  air  from  the  outside,  and  by  means  of  gas  stoves  under  the 
control  of  thermostats — to  keep  the  bath  room  as  nearly  as  possible  at 
the  temperature  of  the  bath;  but  with  the  present  improved  facilities 
for  the  internal  regulation  of  the  baths,  this  is  no  longer  necessary.  The 
recent  practice  is,  in  general,  to  keep  the  temperature  of  the  room  4°  or 
5°  below  that  which  is  to  be  maintained  in  the  bath.  The  flexibility  of 
the  system  of  temperature  regulation,  however,  is  such  that  differences 
of  temperature  amounting  to  25°  can  be  easily  tolerated.  At  the  highest 
temperatures  at  which  osmotic  pressures  have  been  measured,  differ- 
ences of  60°  were  not  infrequent. 

TYPE  IV. 

The  baths  previously  described,  which  are  made  partly  of  wood,  are 
not  adapted  to  the  measurement  of  osmotic  pressure  at  high  temper- 
atures. For  this  purpose,  it  was  necessary  to  construct  baths  of  differ- 
ent design  and  wholly  of  metals. 

The  baths  for  high  temperatures,  which  are  equally  well  adapted  to 
work  at  low  temperatures,  are  of  two  sizes  and  are  made  of  heavy 
sheet  brass  and  copper — mainly  of  the  former.  A  (Figure  38)  exhibits  a 


72 


OSMOTIC  PRESSURE  OP  AQUEOUS  SOLUTIONS. 


section  of  the  inner  compartment  of  one  of  the  smaller  baths.  It  is  this 
compartment  which  is  maintained  at  any  desired  constant  temperature, 
and  in  which  the  cells  are  located  during  a  measurement  of  pressure. 
It  is  circular  in  form  in  the  smaller  baths,  300  millimeters  in  diameter 
and  1  meter  in  height.  Surrounding  this  is  a  large  cylinder  (B,  B) 


38 


r 


FIG.  38. — Brass  and  copper  bath  for  high- 
temperature  work.  Vertical  section. 

(A)  Inner  bath;  (B)  outer  bath;  (L),(L), 
(L),and  (L)  lamps;  (P)  brass  plate  to 
prevent  water  rising  directly  from 
heated  bottom  of  (B)  to  bottom  of  (A) ; 
(Z)  caps  for  lamp  compartments. 


FIG.  39. — Brass  and  copper  bath  for  high  tem- 
peratures, second  vertical  section. 

(A)  Inner  bath;  (O  pumping  tubes;  (E)  space 
into  which  water  coming  up  ((7),  (C)  is  deliv- 
ered; (S),  OS),  and  (Si)  gas  stoves. 


which  is  twice  as  wide  and  much  higher.  In  the  space  between  the  two 
brass  cylinders,  the  water  is  heated  and  made  to  circulate  rapidly  over 
the  exterior  surface  of  the  inner  compartment  (A).  The  circulating 
system  and  the  arrangements  for  heating  the  water  by  gas  are  shown  in 
Figure  39.  C,  C  are  the  pumping  tubes,  100  millimeters  in  diameter, 
which  unite  at  the  top  in  the  short  but  wider  tube  (D) .  At  the  bottom, 
they  open  directly  over  the  gas  stoves  (S,  S) .  There  is  a  third  gas  stove 


MORSE 


PLATE  5 


THE  REGULATION  OF  TEMPERATURE. 


73 


which  is  not  ordinarily  in  use.     The  water,  heated  by  the  stoves 
(S,  £),  is  pumped  up  through  the  tubes  (C,  C)  at  the  rate  of  about  500 
liters  per  minute.     At  the  top  of  D  it  is  delivered  into  the  space  (E,  E), 
whence  it  returns  to  the  bottom  of  the  outer  cylinder  to  be  reheated 
by  the  gas  stoves  and  again  pumped  up  through  the  tubes  (C,  C).     In 
its  downward  course  the  water  passes  over  the  outer  surface  of  A  and 
also  over  the  lamp  compartments  (L,  L,  L,  and  L,  Figure  38).    In  the 
smaller  baths  there  are  6  or  8  of  these  lamp  compartments,  and  in  the 
larger  ones  8  or  12.     They  are  distributed  in  pairs,  one  in  each  pair  being 
located  directly  over  the  other. 
The    circular    openings,   through 
which  the  lamps  are  introduced 
from  the  outside,  are  closed  by 
means  of  caps,  one  of  which  (Z)  is 
shown  in  Figure  38.    Below  the 
inner  compartment  (A,  Figures  38 
and  39)  is  the  disk  (P),  which  pre- 
vents the  water  which  has  been 
heated  by  any  of  the  gas  stoves 
from  rising  directly  against  the 
bottom  of  A.     In  Figure  39,  there 
are  also  to  be  seen  three  tubes, 
indicated  by  dotted  lines,  which 
serve  as  passage  ways  between  the 
exterior  and  interior  for  the  intro- 
duction of  thermometers,  wires,  etc. 
Figure  40  is  a  horizontal  section 
of  one  of  the  smaller  baths,  in 
which  the  positions  of  the  pumping 
tubes  are  indicated  by  C,  (7,  and 
those  of  the  lamps  by  L,  L,  L,  and  L.    The  entrance  to  the  inner  bath 
(A}  150  millimeters  in  width)  is  closed  by  the  plate-glass  door  (7,  Figure 
40)  and  by  the  hollow  metal  door  (M).    There  are  two  such  doors,  as 
will  be  seen  in  Figures  41  and  42.    The  lower  one,  which  is  opened  only 
when  it  is  necessary  to  introduce  or  remove  the  cells,  is  packed  with 
hair.     The  upper  door,  on  the  other  hand,  must  be  opened  whenever  an 
observation  is  to  be  made.    On  this  account,  it  is  provided  with  an  inde- 
pendent temperature-regulating  device,  portions  of  which  can  be  seen  in 
Figure  43.     By  means  of  this,  the  door  is  prevented  from  cooling  down 
when  open.    The  space  between  the  inner  glass  door  and  the  outer  metal 
doors  is  about  40  millimeters  in  width.     It  is  occupied  by  a  brass  frame, 
as  large  as  the  glass  door,  which  is  filled  with  minute  doors,  any  one  of 
which  can  be  opened  independently  of  the  others.    Between  this  frame 
(which  is  placed  close  to  the  glass  door)  and  the  outer  metal  doors  is  a  hair- 
filled  pad,  which  is  so  divided  that  any  small  portion  of  the  frame  of  brass 


FIG.  40. — Brass-copper  bath  for  high  tempera- 
tures. Horizontal  section. 

(A)  Inner  bath;  (C)  and  (C)  pumping  tubes;  (L), 
(L),  (L),  and  (L)  lamp  compartments;  (/) 
plate-glass  door;  (M)  lower  hollow  door  filled 
with  hair. 


74 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


doors  behind  it  may  be  exposed  to  view.  With  these  arrangements, 
and  with  artificial  illumination  by  screened  lamps,  it  is  practicable  to 
observe  objects  in  the  bath  with  very  little  exposure  of  the  interior. 

Figure  41  shows  the  external  appearance  of  the  circular  bath,  and 
Figure  42  the  appearance  of  the  interior  when  a  portion  of  the  outside 
is  removed. 


FIG.  41. — Exterior  view  of  bath  for  high  temperatures. 

FIG.  42. — View  between  interior  and  exterior  baths,  i.  e.,  of  space  filled  with  water. 

Figure  44  is  a  horizontal  section  of  one  of  the  larger  baths  of  Type  IV. 
Like  the  smaller  ones,  they  are  used  for  the  measurement  of  osmotic 
pressure,  and  also  for  all  of  the  purposes  for  which  the  so-called  "man- 
ometer house"  was  formerly  employed — such  as  the  determination  of 
capillary  depression,  the  determination  of  nitrogen  volumes,  the  com- 
parison of  manometers,  etc.  It  is  elliptical  in  form,  the  longer  axis 
being  twice  the  diameter  of  the  compartment  (^l)  in  the  smaller  baths. 
It  is  also  higher  than  the  circular  baths  by  100  millimeters,  giving  a 
height  of  1.1  meters  for  A.  It  has  three  pumping  tubes  (C,  C,  and  C) 
instead  of  two,  and  8  or  12  lamp  compartments  (L,  L,  L,  and  L)  instead 


THE  REGULATION  OF  TEMPERATURE. 


75 


of  6  or  8.  The  two  smaller  metal 
doors  of  the  larger  bath  are  like  the 
corresponding  doors  in  the  smaller 
bath,  except  that  they  are  inserted  in 
a  larger  door,  which  serves  as  a  frame. 
The  relations  of  the  three  doors  will 
be  seen  in  Figures  44  and  45.  The 
largest  door  and  the  lower  small  one 
are  packed  with  hair  and  are  opened 
only  when  it  is  necessary  to  introduce 
or  remove  apparatus,  or  to  make  some 
adjustment  of  the  instruments  within. 
The  upper  of  the  two  smaller  doors, 
which  must  be  opened  whenever  an 
observation  is  to  be  made,  is  provided 
with  a  device  (Figure  43)  for  the  inde- 
pendent regulation  of  its  temperature. 
As  regards  the  disposition  of  the  space 
between  the  glass  and  the  metal  doors, 
there  is  no  difference  between  the  larger  F!G:  43.— Automatic  arrangements  for  main- 


baths  and  the  smaller  ones. 


taining  temperature  of  upper  door  when  open. 


DOOR 


I  LARGE-  //  2  SMALT.  DOORS^  DOOR 
//         in  LARGE-         \ 
//  DOOH. 


Fia.  44. — Larger  (elliptical)  bath  for  high  temperatures.     Horizontal  section. 
(C),  (C),  and  (<7)  pumping  tubes;  (L),  (Li),  (L)  lamp  compartments. 

No  lamps  of  more  than  16  candle-power  are  used  in  baths  of  Type  IV, 
even  when  they  are  maintaining  temperatures  near  the  boiling-point  of 


76 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


water.  For  most  purposes,  lamps  of  8  or  10  candle-power  suffice.  If 
temperatures  but  little  above  that  of  the  room  are  to  be  maintained,  the 
electrical  appliances  only  are  employed  to  heat  and  regulate  the  baths. 
For  low  temperatures,  a  coil  of  block-tin  pipe  is  placed  in  the  space  (E, 
E,  Figure  39),  and  through  it  there  is  made  to  circulate,  under  constant 
pressure,  a  current  of  hydrant  water,  which  has  previously  been  cooled 
with  ice  if  necessary. 


FIG.  45. — Exterior  view  of  larger  bath  for  high  temperatures. 

In  Figure  39  the  thermostat  is  shown  within  the  compartment  (A), 
while  in  Figure  38  it  is  represented  as  being  immersed  in  the  water  of 
the  bath.  The  latter  has  been  found  to  be  the  better  of  the  two  pos- 
sible positions  for  the  instrument.  The  temperature  of  the  water 
about  the  thermostat  may  not  be  precisely  that  of  the  interior  compart- 
ment, and  at  other  points  it  may  vary  slightly,  but  when  the  instrument 
has  once  been  set  for  a  given  interior  temperature,  it  is  quite  capable  of 
maintaining  it  for  any  length  of  time  to  within  0.01°,  which  is  a  rather 
better  regulation  than  can  be  secured  in  baths  of  Type  III. 


CHAPTER  IV. 

THE  MEMBRANES. 

Pfeffer  was  the  first  to  give  to  artificial  semi-permeable  membranes 
the  support  of  a  rigid  background,  and  to  him,  therefore,  belongs  the 
credit  of  having  originated  the  only  practicable  method  of  measuring 
directly  and  correctly  the  osmotic  pressure  of  solutions.  But  many 
years  of  persistent  investigation  were  necessary  in  order  to  overcome 
the  great  difficulties  which  are  inherent  in  the  method  and  to  reduce  it 
to  a  workable  form  through  the  elimination  of  its  sources  of  error. 

The  following  is  an  accurate  restatement,  though  not  an  entirely 
literal  translation,  of  Pfeffer's  description  of  his  method  of  depositing 
membranes : 

"The  clay  cells  were  first  completely  injected  with  water  by  means  of 
repeated  evacuations  under  the  air  pump.  They  were  then  filled  with,  and 
placed  for  several  hours  in,  a  3  per  cent  solution  of  copper  sulphate.  After- 
wards, they  were  several  times  quickly  rinsed  (upon  the  inside  only)  with 
water,  and  well  dried  internally  and  as  expeditiously  as  possible  with  strips 
of  filter  paper.  Having  been  dried  slightly  upon  the  outside,  they  were  left 
exposed  in  the  air  until  the  exterior  surface  was  still  just  moist  to  the  feel. 
The  cells  were  then  filled  with  a  3  per  cent  solution  of  potassium  ferrocyanide, 
and  returned  to  the  solution  of  copper  sulphate. 

"  After  standing  quietly  from  24  to  48  hours,  the  cells  were  filled  completely 
with  a  solution  of  ferrocyanide  and  closed.  The  contents  now  developed  grad- 
ually a  certain  over-pressure  due  to  the  superior  osmotic  pressure  of  the  interior 
solution.  After  another  period  of  from  24  to  48  hours,  the  apparatus  was 
opened  and  refilled  with  a  solution  containing  3  per  cent  of  potassium  ferro- 
cyanide and  1.5  per  cent  of  saltpeter  (by  weight),  which  developed  ordinarily 
an  osmotic  pressure  of  something  more  than  3  atmospheres.  If  the  cells  were 
to  be  used  for  higher  pressures,  they  were  tested  with  solutions  containing 
more  saltpeter." 

Pfeffer  found  that  a  "slow  increase  in  pressure  and  a  certain  period 
of  low  pressure"  were  essential  to  success  in  the  preparation  of  his  mem- 
branes. The  explanation  which  he  gives  is  that,  at  certain  points,  the 
membrane  spans  depressions  in  the  cell  wall,  and  is,  therefore,  more 
liable  to  rupture,  if  the  pressure,  which  is  to  force  the  membrane  against 
the  wall  in  such  unsupported  places,  is  rapidly  or  suddenly  increased. 
He  also  states  that  a  somewhat  prolonged  period  of  deposition  is  neces- 
sary in  order  to  give  to  the  membrane  the  strength  which  will  enable  it 
to  withstand  pressure. 

The  attempts  which  were  made  in  this  laboratory  many  years  ago  to 
reproduce  the  membranes  of  Pfeffer  with  a  view  to  measuring  osmotic 
pressure,  especially  that  of  concentrated  solutions,  were  failures,  as 
have  been  all  similar  attempts  on  the  part  of  other  investigators.  It 
was  not  impossible  to  make  membranes  which  would  yield  impressive 

77 


78        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

osmotic  phenomena,  but  when  they  were  tested  as  to  their  sufficiency 
for  quantitative  purposes  by  the  rule  that  a  perfect  membrane  must  be 
able  to  develop  and  maintain  maximum  pressures,  without  leakage  of 
the  solute  at  any  time  during  an  experiment,  they  were  found  to  be 
wanting. 

The  failure  of  the  membranes  to  justify  the  hopes  which  had  been 
entertained  of  them  could  be  ascribed  to  any  one,  or  all,  of  several 
causes — to  the  want  of  perfect  semi-permeability  on  the  part  of  the 
copper  ferrocyanide;  to  the  faulty  character  of  the  supporting  porous 
wall;  or  to  the  imperfect  attachment  of  the  membrane  to  the  wall;  or  to 
all  three  of  these  possible  defects.  It  was  strongly  suspected  that  the 
third  cause  for  failure  existed;  in  other  words,  that  the  membranes  made 
by  the  method  of  Pfeffer  are  not  at  all  points  firmly  attached  to  the 
supporting  wall. 

The  ideal  membrane  (as  regards  location)  is  obviously  one  consisting 
of  innumerable  plugs  which  are  driven  so  firmly  into  the  mouths  of  the 
pores  which  open  on  the  interior  surface  of  the  cell  that  no  pressure  can 
rupture  or  dislodge  them — that  is,  the  membrane  must  be  firmly 
embedded  in  the  wall  and  not  consist  of  a  mere  (more  or  less  detached) 
cover  for  its  interior  surface.  Looked  at  from  this  point  of  view,  the 
practice  of  Pfeffer  of  carefully  drying  the  interior  of  his  cells  with  filter 
paper  was  a  highly  rational  procedure.  Subsequent  observations  have 
proved  it  to  be  the  vital  feature  of  his  process.  Pfeffer  himself  recog- 
nized this  in  a  practical  way,  but  he  appears  to  have  regarded  these 
membranes  as  "aufgelagert"  rather  than  embedded,  though  he  gives  no 
explanations;  for  he  says  (page  9),  "wdhrend  ich  anfangs  mit  grossen 
Schwierigkeiten  zu  kdmpfen  hdtte,  und  ehe  ich  zu  partielkr  Abtrocknung 
meine  Zuflucht  nahm,  uberhaupt  keine  aufgelagerte  Membran  zu  Stande 
brachte."  The  obvious  purpose  of  the  "Abtrocknung"  was  to  empty  the 
mouths  of  the  pores  of  the  solution  of  copper  sulphate,  in  order  that  they 
might  be  filled  with  the  solution  of  potassium  ferrocyanide  and  thus 
force  the  formation  of  an  embedded  membrane.  It  seemed  probable, 
however,  that  the  procedure  of  Pfeffer  failed,  in  some  degree,  to  accom- 
plish its  purpose,  i.e.,  that  the  embedding  was  emperfect;  and  the  ques- 
tion arose  whether  it  might  not  be  possible  to  devise  some  method  by 
means  of  which  these  "plugs,"  of  which  the  membrane  should  exclu- 
sively consist,  could  be  more  firmly  driven  into  and  more  securely  fixed 
in  their  places  than  is  the  case  when  the  diffusion  method  of  Pfeffer  is 
employed.  It  was  in  this  connection  that  the  electrolytic  process  for 
the  deposition  of  the  membranes  occurred  to  the  writer. 

It  should  be  stated,  however,  that  the  first  suggestion  which  led  up  to 
the  solution  of  the  problem  came  accidentally.  While  the  subject  of 
measuring  osmotic  pressure  was  still  uppermost  in  the  mind  of  the 
writer,  he  was  engaged  in  an  attempt  to  procure  pure  aqueous  solutions 
of  permanganic  acid  by  the  electrolysis  of  potassium  permanganate. 


THE   MEMBRANES.  79 

The  process  consisted  in  placing  two  porous  cups,  partly  filled  with 
water,  side  by  side  in  a  solution  of  the  salt,  and  electrolyzing  with  the 
anode  in  one  cup  and  the  cathode  in  the  other.  At  times  the  pores  of 
the  anode  cup  became  filled  with  a  deposit  of  peroxide,  and  it  was 
noticed  that  whenever  this  occurred  the  volume  of  the  contents  of  that 
cell  seemed  to  increase  more  rapidly  than  could  be  accounted  for  by 
any  probable  "electrical  endosmose"  of  the  solvent.  It  was  imme- 
diately suspected  that  the  deposit  of  peroxide  in  the  porous  wall  was 
playing  the  part  of  a  semi-permeable  membrane  and,  upon  further 
investigation,  the  suspicion  was  proved  to  be  well  founded.  The  pro- 
cess by  which  the  semi-permeable  peroxide  was  deposited  in  the  pores 
of  the  cell  and  that  by  which  osmotic  membranes  are  now  made  differ 
radically;  but  the  mere  fact  of  having  formed  one  active  membrane  by 
electrolytic  means  sufficed  to  suggest  the  practicability  of  employing 
electricity  for  the  production  of  all  semi-permeable  membranes. 

The  work  of  Pfeffer  was  unique  and  brilliant,  and  its  consequences 
have  been  far-reaching  and  beneficent.  The  writer,  after  fourteen 
years  of  activity  in  the  same  difficult  field,  has  more  reason  than  any 
other  to  appreciate  its  great  merit  and  less  justification  than  any  other 
for  detracting  from  its  value.  This  statement  of  attitude  is  made  with 
a  view  to  disarming  any  possible  suspicion  of  careless  criticism  on  the 
part  of  the  writer,  when  he  states,  on  the  basis  of  his  own  experience, 
that  none  of  the  pressures  recorded  by  Pfeffer  could  have  been  the  maximum 
pressures  of  his  solutions. 

The  final  step  in  the  preparation  of  Pfeffer's  cells  for  quantitative 
measurements  was  by  means  of  a  solution  containing  3  per  cent  of 
potassium  ferrocyanide  and  1.5  per  cent  of  potassium  nitrate.  The 
cells  were  filled  with  this  solution,  closed,  and  placed  in  3  per  cent  solu- 
tions of  copper  sulphate.  Pfeffer  states  that  the  pressure  subsequently 
developed  was  usually  something  more  than  3  atmospheres.  If  we 
add  the  excess  of  the  ferrocyanide's  pressure  (over  that  of  the  copper 
sulphate)  to  the  true  pressure  of  the  nitrate,  the  total  pressure  which 
should  have  been  developed  is  something  more  than  6.5  atmospheres. 
The  unavoidable  conclusion  is  that  the  membranes  did  not  perfectly 
retain  the  solute,  and  that  the  subsequent  measurements  were  under- 
taken with  defective  cells. 

In  order  to  show  that  the  osmotic  pressure  of  solutions  obeys  the  law 
of  Boyle  for  gases,  van't  Hoff  cites  the  pressures  of  the  1,  2,  4,  and  6  per 
cent  solutions  of  cane  sugar  which  were  obtained  by  Pfeffer.  These 
pressures  are  strikingly  proportional  to  the  concentration,  which  is  the 
form  of  the  law  as  applied  to  solutions.  There  was,  at  that  time,  and  for 
many  years  thereafter,  no  reason  known  why  this  evidence  should  not 
be  accepted  at  its  face  value,  and  its  validity  appears  never  to  have  been 
questioned;  accordingly  one  finds  it  repeated  and  emphasized  in  every 
presentation  of  the  subject  of  osmotic  pressure  from  1887  to  the  present 


80 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


time.  The  validity  of  the  proof  depends,  however,  upon  the  question 
whether  the  pressures  obtained  by  Pfeffer  were  the  full  (maximum) 
pressures  of  his  solutions.  Pfeffer  believed  them  to  be  so;  for  he  men- 
tions having  ascertained,  by  means  of  specific-gravity  determinations, 
that  the  solutions  did  not  lose  appreciably  in  concentration  while  in  the 
cells.  It  is  questionable,  however,  if  this  method  is  sufficiently  delicate, 
unless  carried  out  with  extraordinary  precautions,  to  detect  differences 
which,  if  discovered,  might  well  have  led  to  a  verdict  unfavorable  to  the 
applicability  of  Boyle's  law.  The  writer  and  his  associates  have  not 
yet  exactly  repeated  the  experiments  of  Pfeffer,  though  they  expect  to 
do  so  in  the  near  future.  They  have,  however,  done  enough  work  at 
temperatures  and  concentrations  approximating  to  those  of  Pfeffer  to 
enable  them  to  predict  with  considerable  confidence  about  what  the 
pressures  in  question  will  be  found  to  be.  The  following  table  gives 
the  pressures  found  by  Pfeffer  which  have  been  universally  quoted  as 
proof  of  the  conformity  of  osmotic  pressure  to  Boyle's  law  and  those 
which  the  work  of  the  writer  and  his  associates  enable  them  to  predict : 

TABLE  3. 


1. 

2. 

3. 

4. 

5. 

Concentration. 
Grams  of  sugar 
per  1000  c.c. 
H,0. 

Pressures 
found  by 
Pfeffer. 

Pressures 
required  by 
Boyle's  law 
at  15°. 

Pressures 
Pfeffer  should 
have  found. 

Differences 
between 
Pfeffer's  and 
the  author's 
results. 

10 

0.71  atms. 

0.69  atms. 

0.75  atms. 

5.6  per  cent. 

20 

1.34 

1.37 

1.48 

10.4 

40 

2.74 

2.77 

3.00 

9.5 

60 

4.05 

4.16 

4.41 

8.8 

If  we  compare  the  pressures  found  by  Pfeffer  with  those  required  by 
the  law  of  Boyle  at  15° — that  is,  columns  2  and  3 — the  agreement  is 
astonishing,  and  it  is  not  surprising  that  these  data  have  played  an 
important  part  in  nearly  all  discussions  of  osmotic  pressure  during  the 
last  twenty-five  years.  If,  on  the  other  hand,  we  compare  the  pres- 
sures of  Pfeffer  with  those  calculated  for  the  same  solutions  from  the 
results  of  the  author  and  his  associates — that  is,  columns  2  and  4 — it 
will  be  observed  that  the  latter  are  considerably  higher  than  the  former. 
The  differences  (column  5)  vary  from  5.6  to  10.4  per  cent.  The  data 
given  in  column  4,  which  are  designated  (somewhat  presumptuously 
perhaps)  as  "the  pressures  which  Pfeffer  should  have  found,"  are  all 
calculated  from  measurements  made  after  the  method  of  measuring  had 
been  brought  to  its  highest  state  of  perfection — that  is,  after  the  last 
vestige  of  leakage  of  solute  had  been  eliminated.  The  inference  to  be 
drawn  from  the  facts,  as  stated  above,  is  that  the  smaller  pressures  of 
Pfeffer  were  due  to  some  leakage  of  the  solute.  The  solutions  of  Pfeffer 
which  are  cited  in  this  discussion  contained  in  1,000  cubic  meters  of 


THE   MEMBRANES.  81 

water  10,  20,  40,  and  60  grams  of  sugar;  while  those  from  whose  pres- 
sures the  values  given  in  column  4  were  calculated^contained  in  the 
same  volume  of  solvent  8.49, 16.98,  33.96,  and  67.92  grams.  They  were, 
in  fact,  0.025, 0.050, 0.100,  and  0.150  "weight-normal"  solutions,  whose 
pressures  were  0.64,  1.28,  2.54,  and  4.99  atmospheres  respectively. 

To  one  who  has  had  long  experience  in  the  measurement  of  osmotic 
pressure,  the  conformity  of  Pfeffer's  results  to  the  requirement  of  Boyle's 
law — stated  merely  as  proportionality  of  pressure  to  concentration — is 
not  surprising.  It  is,  in  fact,  almost  a  necessary  consequence  of  using 
cells  which  do  not  quite  perfectly  retain  the  solute.  The  leakage  of 
solute  from  any  series  of  defective,  but  equally  good,  cells  is  propor- 
tional to  the  pressures  of  the  solutions  rather  than  to  their  supposed 
concentrations.  In  other  words,  the  pressures  of  cane-sugar  solutions, 
between  0°  and  25°,  are  for  some  reason  not  proportional  to  their  sup- 
posed concentration.  If  now  a  series  of  them  of  varying  strength  are 
placed  in  defective  cells,  the  escape  of  solute  will  be  proportional  to  the 
pressures,  and  not  to  the  supposed  relative  concentration  of  the  solu- 
tions. The  necessary  consequence  is  that  the  relative  pressures  will 
gradually  approach  proportionality  to  the  (supposed)  relative  concen- 
trations of  the  solutions.  In  a  later  chapter  the  author  will  have  occa- 
sion to  show  how  membranes  which  do  not  leak  may  be  the  means  of 
appearing  to  establish  the  applicability  of  Boyle's  law  to  osmotic  pres- 
sure upon  evidence  which  is  superficially  convincing,  but  fundamentally 
unsound.  It  may  be  stated  here  that  all  solutions  of  cane  sugar  thus 
far  investigated  do  obey  the  law  of  Gay-Lussac  between  0°  and  25°, 
while  none  of  them  appear  to  obey  the  law  of  Boyle  until  higher  temper- 
atures are  reached.  That  this  failure,  in  the  latter  case,  may  be  more 
apparent  than  real  will  be  shown  elsewhere. 

It  is  fortunate  that  the  validity  of  the  generalizations  of  van't  Hoff 
regarding  solutions  does  not  depend  exclusively  upon  the  correctness  of 
Pfeffer's  measurements;  and,  in  one  sense,  it  is  also  fortunate  that 
Pfeffer  obtained  the  results  which  he  did,  rather  than  the  correct  pres-  • 
sures  of  his  solutions;  for,  whatever  may  have  been  the  relative  impor- 
tance assigned  to  them  by  van't  Hoff,  they  undoubtedly  contributed 
more  than  any  other  one  of  his  arguments  to  the  immediate  and  general 
acceptance  of  his  views. 

The  first  announcement*  regarding  the  electrolytic  method  of  deposit- 
ing membranes  was  made  in  the  following  words: 

"If  a  solution  of  a  copper  salt  and  one  of  potassium  ferrocyanide  are  sepa- 
rated by  a  porous  wall  which  is  filled  with  water,  and  a  current  is  passed  from 
an  electrode  in  the  former  to  another  electrode  in  the  latter  solution,  the  copper 
and  the  ferrocyanogen  ions  should  meet  within  the  wall  and  separate  as  copper 
ferrocyanide  at  all  points  of  meeting,  so  that  in  the  end  there  should  be  built 
up  a  continuous  membrane  well  supported  on  either  side  by  the  material  of 
the  wall." 

*Amer.  Chem.  Journal,  xxvi,  81. 


82        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

The  electrolytic  method  of  depositing  membranes  was  soon  success- 
fully applied  to  all  the  usual  varieties  of  porous  vessel  which  accumulate 
in  a  laboratory,  and  to  several  new  forms  of  the  same.  It  was  found, 
in  fact,  that  a  highly  active  membrane  could  be  produced  without  diffi- 
culty in  every  kind  of  porous  wall.  The  method  was  also  employed 
for  the  deposition  in  porous  vessels  of  many  other  compounds  than 
copper  ferrocyanide,  and  a  large  number  of  these  (about  25)  proved  to 
be  osmotically  active. 

The  success  of  the  new  method  as  a  means  of  building  up  membranes 
was  placed  beyond  question,  but  the  possession  of  it  did  not  enable  us  to 
proceed  at  once  to  the  measurement  of  osmotic  pressure.  The  resisting 
power  of  the  membranes  made  by  the  electrolytic  method  was  much 
greater  than  that  of  those  produced  by  the  process  of  Pfeffer,  but  even 
they  were  unable,  in  the  porous  vessels  then  available,  to  withstand  high 
pressures  without  rupture.  Our  attention  during  the  succeeding  four 
years  was  given  almost  exclusively  to  the  porous  wall  on,  or  within, 
which  the  membrane  is  deposited. 

A  brief  account  of  this  part  of  the  investigation  has  been  given  in  the 
first  chapter;  but  a  concise  restatement  of  those  structural  character- 
istics of  the  cell  wall,  upon  which  the  efficiency  of  the  membrane  was 
found  to  depend,  will  be  useful  in  this  place.  They  are : 

1.  A  great  and  uniform  strength  of  wall,  which  was  secured  by 

employing  mixtures  of  different  clays. 

2.  Absence  of  "air  blisters,"  which  were  eliminated  by  subjecting 

the  clays  to  great  pressures. 

3.  An  exceedingly  fine  and  uniform  porosity,  which  was  secured  by 

the  employment  of  the  finest  portions  only  of  the  clays,  by 
subjecting  the  mixtures  to  high  pressure,  and  by  burning  at 
high  temperatures. 

THE  DEPOSITION  OF  THE  MEMBRANE. 

.  The  first  steps  toward  the  formation  of  the  membrane  are  the  expul- 
sion of  air  from  the  pores  of  the  cell  and  its  replacement  by  water.  This 
has  been  effected  from  the  beginning  by  means  of  the  considerable  vol- 
ume of  water  transported  by  the  cations  whenever  dilute  aqueous  solu- 
tions of  salts  are  subjected  to  electrolysis.  At  first  the  salt  employed 
was  potassium  sulphate,  but  the  fact  that  the  "atmosphere"  of  water 
which  surrounds  the  lithium  ion  is  much  greater  than  that  transported 
by  the  potassium  ion  suggested  the  use  of  lithium  salts  rather  than 
those  of  potassium.  A  series  of  quantitative  comparisons  carried  out 
by  Frazer  showed  that  the  quantities  of  water  drawn  through  the  cell 
wall  conform  to  the  following  rule: 

The  volumes  of  water  carried  through  the  porous  wall  of  a  cell,  under 
identical  conditions,  are  inversely  proportional  to  the  relative  velocities  of 
the  various  cations,  divided  by  their  respective  valencies. 


THE   MEMBRANES.  83 

The  improvement  in  the  method  which  followed  the  replacement  of 
the  potassium  salt  by  lithium  sulphate  was  very  striking.  A  0.005  nor- 
mal solution  was  employed.  The  cell  is  nearly  filled  with  the  solution 
and  immersed  in  the  same  to  the  lower  limit  of  the  glazed  portion.  The 
electrodes  are  of  platinum  and  the  one  within  the  cell  is  made  the 
cathode.  Provision  is  made  for  the  automatic  removal  of  the  water 
which  is  drawn  into  the  cell  through  the  pores.  At  intervals  the  elec- 
trolysis is  interrupted  for  the  purpose  of  mixing  the  liquid  which  has 
been  removed  by  the  siphon  with  the  solution  in  the  outer  vessel. 
When  it  is  thought  that  all  the  air  has  been  expelled  from  the  pores,  the 
cell  is  taken  out,  emptied,  and  rinsed  with  pure  water;  it  is  then  soaked 
for  a  time  in  distilled  water,  which  is  frequently  renewed;  lastly,  it  is 
filled  with,  and  partially  immersed  in,  pure  water,  and  the  electrolysis  is 
resumed.  When  the  conductivity,  after  frequent  renewals  of  the  water, 
has  fallen  nearly  to  that  which  is  normal  for  the  distilled  water,  the  cell 
is  ready  for  the  deposition  of  the  membrane.  If  the  membrane  is  not  to 
be  deposited  immediately,  the  cell  is  placed,  and  kept  until  needed,  in 
water  in  which  a  little  thymol  or  formaldehyde  has  been  dissolved.  The 
reason  for  this  precaution  will  appear  later  when  the  subject  of  the  infec- 
tion of  the  membrane  is  taken  up.  It  is  also  well  to  take  any  other  pre- 
cautions against  infection  which  suggest  themselves,  such  as  boiling  all 
the  water  which  comes  in  contact  with  the  cells,  covering  the  vessels  in 
which  they  are  kept,  etc.  Such  precautions  are  by  no  means  superfluous. 

The  arrangement  for  the  deposition  of  the  membrane  is  as  follows: 
the  anode,  which  consists  of  a  cylinder  of  copper,  nickel,  or  cobalt, 
according  to  the  composition  of  the  membrane  to  be  deposited,  is  placed 
in  an  empty  glass  vessel,  and  within  the  cylinder  is  placed  the  cell, 
which  is  closed  by  a  rubber  stopper  carrying  (1)  the  cathode,  a  platinum 
cylinder;  (2)  a  funnel  with  a  stem  nearly  long  enough  to  reach  the  bottom 
of  the  cell;  and  (3)  an  overflow  tube.  The  circuit  is  closed,  and,  as 
nearly  simultaneously  as  possible,  the  cell  and  the  vessel  outside  of  it 
are  filled,  each  with  its  appropriate  solution.  The  solutions,  which  in 
the  majority  of  the  experiments  to  be  reported  are  potassium  or  lithium 
ferrocyanide,  and  copper  or  nickel  sulphate,  are  made  one-tenth  normal. 
The  voltage  employed  is  1 10.  At  first  the  resistance  is  very  high,  owing 
to  the  fact  that  the  cell  wall  is  filled  with  nearly  pure  water.  Very  soon, 
howrever,  the  current  begins  to  increase,  and  within  a  short  time  it 
attains  a  maximum.  It  then  drops  steadily  for  two  or  three  hours,  and 
perhaps  longer,  when  it  reaches  a  minimum  corresponding  to  the  maxi- 
mum resistance  of  the  membrane  which  it  is  possible  to  obtain  at  that 
"running."  If  the  electrolysis  is  continued  very  long  after  the  current 
has  reached  a  minimum,  the  resistance  begins  to  fall  again,  and  the 
decline  persists  until  the  circuit  is  broken.  This  strange  behavior  of 
the  membrane  appears  to  have  some  connection  with  an  accumulation 
of  alkali  in  the  cell  and  perhaps  in  the  membrane  itself;  accordingly, 


84       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

during  the  electrolysis,  the  ferrocyanide  is  renewed  every  2  or  3  min- 
utes by  pouring  a  fresh  solution  of  it  into  the  funnel.  A  temporary 
increase  in  resistance  follows  each  renewal,  but  this  may  be  due  in  part 
to  a  fall  in  the  temperature  of  the  contents  of  the  cell.  The  final  decline 
in  resistance  may  be  postponed  by  frequent  renewals  of  the  solution,  but 
not  indefinitely.  For  this  reason,  it  is  suspected  that  the  phenomenon 
is  possibly  due  to  accumulation  of  alkali  in  the  membrane.  Having 
reached  its  maximum  resistance,  the  membrane  can  not  be  further 
improved  by  electrolysis,  without  a  thorough  preliminary  soaking  in 
pure  water — that  is,  water  which  is  free  from  electrolytes.  The  cell  is 
therefore  placed  in  water  in  which  a  little  thymol  has  been  dissolved, 
and  is  allowed  to  soak,  with  frequent  renewals  of  the  water,  for  several 
days.  The  period  of  soaking  is  quite  indefinite,  but  experience  has 
shown  that  it  should  be  not  less  than  3  days;  and  that,  in  general, 
the  longer  the  soaking  is  the  better  will  be  the  result  of  the  succeeding 
electrolysis. 

After  soaking,  not  less  than  3  days,  the  membrane-forming  process  is 
repeated.  On  this  occasion  the  resistance  usually  rises  much  higher 
than  before,  but  finally  reaches  a  maximum  beyond  which  it  can  not  be 
driven,  however  frequently  the  solution  of  ferrocyanide  may  be  renewed. 
If  the  electrolysis  is  continued  beyond  this  point,  a  gradual  fall  in  resist- 
ance sets  in,  which  may  eventually  lead  to  the  total  ruin  of  the  mem- 
brane. As  in  the  first  instance,  when  it  is  found  that  the  resistance  no 
longer  increases,  the  electrolysis  is  interrupted  and  the  cell  is  again 
placed  in  water  for  a  period  of  3  or  more  days.  The  further  procedure 
is  simply  a  repetition  of  the  alternate  "running"  and  "soaking"  described 
above,  which  is  persisted  in  until  the  resistance  of  the  membrane  can  be 
forced  no  higher.  The  maximum  resistance  which  is  finally  obtained 
varies  greatly.  Obviously,  it  varies  with  the  effective  area  of  the 
membrane;  and  this,  in  turn,  varies  from  cell  to  cell,  according  to  the 
porosity  of  the  cell  wall.  In  general,  it  is  found  that  membranes  in 
hard-burned  cells  have  high  final  resistances.  The  temperature  of  depo- 
sition is  also  an  important  factor  in  determining  resistance.  In  a  given 
cell,  a  membrane  deposited  or  repaired  at  0°  may  have  a  resistance  of 
more  than  1,000,000  ohms,  while  at  80°  the  resistance  can  not  be  driven 
above  1,000  ohms.  Again,  the  resistance  of  the  membranes  increases 
with  age  and  repeated  use. 

Having  developed  the  maximum  resistance  in  the  manner  described, 
the  membrane  is  subjected  to  a  process  of  "seasoning"  under  pressure. 
For  this  purpose  the  cell  is  set  up  with  a  concentrated  solution  of  cane 
sugar  (not  less  than  half  normal)  to  which  a  small  amount  of  ferro- 
cyanide has  been  added,  an  osmotically  equivalent  quantity  of  copper 
sulphate  having  been  dissolved  in  the  water  in  which  the  cell  is  to 
stand  during  the  experiment.  The  initial  mechanical  pressure  which  is 
brought  upon  the  contents  of  the  cell  at  the  time  of  closing  may  be 


THE   MEMBRANES.  85 

above  the  known  osmotic  pressure  of  the  solution  or  considerably  below 
it.  In  the  former  case  the  mercury  in  the  manometer  falls,  in  the  latter 
it  rises.  Eventually,  in  either  case,  the  meniscus  generally  comes  to 
rest  at  a  point  below  the  position  it  should  have,  showing  that  the  solu- 
tion has  been  diluted.  The  irregular  movements  of  the  meniscus, 
which  often  precede  the  assumption  of  its  final  position,  are  all  such  as 
can  be  interpreted  as  being  due  to  a  breaking  of  the  membrane  under 
pressure,  and  a  mending  of  the  rents  by  the  membrane-formers.  What- 
ever may  be  the  result  of  the  first  trial,  which  usually  extends  over  sev- 
eral days  before  the  pressure  becomes  constant,  the  cell  is  emptied  and 
soaked  from  3  days  to  a  week  in  distilled  water,  when  it  is  again  sub- 
jected to  the  membrane-forming  process.  Afterwards,  it  is  again  set 
up  with  a  solution  of  cane  sugar  in  the  same  manner  as  for  the  first  trial. 
On  the  second  trial  the  pressure  developed  is  usually  higher  than  on  the 
first.  Sometimes,  but  not  often,  the  full  osmotic  pressure  of  the  un- 
diluted solution  is  obtained.  The  further  procedure  with  the  cell  is 
simply  a  repetition  of  the  steps  already  described.  Sooner  or  later  there 
is  developed  in  this  way  a  cell  which  gives  maximum  pressures  on  every 
occasion,  and  in  which  the  solutions  suffer  no  dilution.  It  is  then  ready 
for  the  measurement  of  osmotic  pressure. 

OBSERVATIONS  ON  THE  MEMBRANE. 
1.  TEMPERATURE  OF  DEPOSITION. 

As  nearly  as  possible,  the  membrane  is  deposited  and  developed  at  the 
temperature  at  which  it  is  afterwards  to  be  employed  for  the  measure- 
ment of  pressure.  The  temperature  rises  somewhat  during  the  deposi- 
tion, but  is  kept  within  limits  by  the  frequent  renewals  of  the  ferro- 
cyanide  solution.  During  the  intervals  of  rest,  i.  e.,  while  soaking,  the 
cell  is  also  maintained  at  the  temperature  at  which  its  membrane  was 
formed  and  at  which  it  is  to  be  used  for  the  measurement  of  pressure. 

When  a  cell  has  been  prepared  or  used  at  one  temperature  and  is  to 
be  prepared  for  use  at  another,  the  ease  with  which  the  change  may  be 
accomplished  depends  very  much  upon  the  relation  of  the  different  tem- 
peratures to  one  another,  and  whether  they  are,  in  general,  high  or  low 
temperatures.  It  has  been  found  that  when  the  temperatures  in  ques- 
tion are  moderate  ones,  e.  g.,  between  0°  and  30°,  it  is  better  to  deposit 
the  membrane  and  use  the  cell  at  the  highest  temperature  first,  and  then 
to  work  at  each  of  the  lower  temperatures  in  the  descending  order.  The 
reverse  order  is,  however,  entirely  practicable.  If  the  membranes  are 
to  serve  for  measurements  at  high  temperatures,  e.  g.,  50°  to  90°,  the 
training  of  the  cells  for  their  work  is  quite  laborious.  It  is  then  neces- 
sary to  deposit  and  "train"  the  membranes  at  some  moderate  temper- 
ature, e.  g.,  30°,  and  to  repeat  these  operations  at  short  temperature- 
intervals  in  the  ascending  order. 


86        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

A  cell  which  has  been  used  at  a  high  temperature,  and  has  then  been 
allowed  to  cool  rapidly  and  to  stand  at  a  considerably  lower  temper- 
ature, is  generally  ruined  for  use  at  any  temperature  whatsoever.  An 
unfortunate  experience  of  the  writer  and  one  of  his  associates  will  illus- 
trate the  point.  Starting  with  about  25  cells  at  25°,  they  had  labor- 
iously trained  them  up  by  short  temperature-intervals,  until  they  were 
measuring  with  perfect  success  at  70°  and  80°.  When  the  summer 
vacation  arrived,  the  cells  were  put  in  soak  in  thymol  water,  as  usual  at 
such  times,  and  allowed  to  stand  through  the  summer  months.  On 
resuming  the  work  in  the  autumn,  it  was  found  impossible  to  restore  the 
cells  to  a  usable  condition  at  high  temperatures,  and  only  a  few  of  them 
were  afterwards  useful  at  any  temperature.  It  is  not  known  what  hap- 
pened to  the  membranes  in  consequence  of  the  large  and  rather  rapid 
temperature  transition.  It  is  possible  that  they  might  have  been  saved 
by  reversing  the  process  by  which  they  were  built  up  for  use  at  high 
temperatures,  i.  e.,  by  dropping  the  temperature  gradually  and  "season- 
ing" the  cells  into  a  usable  condition  at  short  temperature-intervals. 
Membranes  which  have  been  prepared  and  used  at  any  ordinary  tem- 
perature withstand  the  fluctuations  of  summer  temperature  without 
deterioration. 

2.  TREATMENT  OF  THE  CELL  WHILE  IN  USE. 

The  foregoing  statements  have  especial  reference  to  the  preparation 
of  the  cell  for  the  measurement  of  pressure.  The  treatment  of  the  cell 
while  in  use  has  not  been  stated  in  sufficient  detail.  Having  built  up 
the  membrane,  in  the  manner  described,  until  its  resistance  can  be 
forced  no  higher,  and  having  afterwards  " seasoned"  it  under  pressure 
until  the  solutions  are  proved  to  suffer  no  dilution  while  in  the  cell, 
the  formal  measurements  of  osmotic  pressure  are  begun.  The  first 
statement  to  be  made  in  this  connection  is  the  general  one  that,  from 
the  time  the  deposition  of  the  membrane  is  commenced  until  the  work 
of  measuring  at  a  given  temperature  is  finished,  the  cell  is  maintained, 
as  nearly  as  possible,  at  temperature.  This  makes  it  necessary  also 
to  maintain  at  temperature  all  the  solutions  which  are  used  with  it. 

Following  the  custom  of  Pfeffer,  there  is  added  to  the  solution  whose 
pressure  is  to  be  measured  a  small  amount  of  potassium  ferrocyanide. 
The  exact  quantity  is  83.9  milligrams  to  each  100  grams  of  water, 
which  gives  a  0.01  weight-ion-normal  solution,  if  the  dissociation  of 
the  salt  is  complete.  This  solution  is  one-tenth  as  strong,  with  respect 
to  the  ferrocyanide,  as  that  formerly  used  in  depositing  the  membranes, 
and  is  of  the  same  strength  as  that  which  is  employed  in  developing 
them  under  pressure.  An  osmotically  equivalent  quantity  of  copper 
sulphate  (123.9  milligrams  per  100  grams  of  water)  is  added  to  the 
water  in  which  the  cell  stands  during  an  experiment.  The  solutions 
without  and  within  the  cell  are  also  made  0.001  weight-normal  with 
thymol  to  guard  against  infection.  The  presence  of  the  "membrano- 


THE   MEMBRANES.  87 

gens,"  as  they  are  called  by  Pfeffer,  is  undoubtedly  of  service  while 
the  development  of  the  membranes  under  pressure  is  in  progress;  but 
it  is  still  a  question  whether  they  are  required  after  the  membranes 
have  once  been  perfected. 

After  finishing  a  measurement  of  pressure,  the  cell  is  emptied,  is 
thoroughly  washed,  and  is  then  allowed  to  soak  three  days  or  more  in 
water  which  is  0.001  normal  with  respect  to  thymol.  The  water  is 
renewed  at  least  twice  each  day.  The  cell  is  then  ready  to  be  prepared 
for  another  measurement  of  pressure.  The  preparation  consists  in  sub- 
jecting it  to  one  or  more  repetitions  of  the  membrane-forming  process, 
until  the  high  resistance  of  the  membrane  indicates  that  its  condition  is 
again  satisfactory. 

3.  THE  SOAKING  OF  THE  CELL. 

It  has  previously  been  stated  that  before  every  repetition  of  the 
membrane-forming  or  repairing  process,  the  cell  (whether  it  is  being 
prepared  for  use  or  is  in  use)  is  soaked  in  distilled  water  for  a  consider- 
able period.  This  treatment  is  of  the  greatest  importance — in  fact, 
it  can  not  be  dispensed  with.  Moreover,  it  may  be  stated,  as  a  general 
proposition,  that  the  longer  the  soaking  is  continued  the  better  will 
the  condition  of  the  membrane  be  found  to  be.  In  accord  with  this 
statement  is  the  fact  that  those  membranes  which  have  soaked  through 
the  three  summer  months  without  interruption  are  always  found  to 
be  in  excellent  condition  for  the  resumption  of  work  in  the  autumn. 
The  statement  does  not,  of  course,  apply  to  cells  which  have  suffered 
from  infection,  or  from  the  effects  of  use  with  electrolytes,  or  from  use 
at  high  temperatures  followed  by  too  rapid  cooling.  The  observed  effect 
of  too  little  soaking  is  always  an  inability  on  the  part  of  the  membrane 
to  maintain  pressure. 

The  beneficial  effect  of  water  on  the  membranes  is  not  fully  under- 
stood, but  it  is  believed  to  be  due  to  the  extraction  of  alkaline  metals 
from  the  membrane  material,  and  therefore  to  the  effect  which  such 
extraction  may  be  supposed  to  have  in  the  preservation  or  improve- 
ment of  the  colloidal  condition  of  the  membranes.  It  is  the  purpose 
of  the  writer,  while  engaged  upon  this  investigation,  to  confine  himself, 
as  much  as  possible,  to  the  discussion  of  established  facts,  but  he 
ventures  to  suggest  in  the  present  connection  that  all  the  phenomena 
which  have  come  under  his  observation  are  in  accord  with  the  idea 
that  true  semipermeability  is  an  attribute  of  colloids  only,  and  that 
the  passage  of  water  through  an  osmotic  membrane  is  a  phenomenon 
of  the  hydration  of  a  colloid  upon  one  side  and  its  partial  dehydration 
on  the  other.  This  statement  explains  nothing,  but  it  enables  one 
to  account,  in  a  plausible  manner,  for  the  highly  beneficial  effect  of 
pure  water  upon  the  semipermeability  of  membranes,  and  also  for  the 
deleterious  effect  of  electrolytes. 


88        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

The  suspicion  that  much  of  the  difficulty  experienced  in  securing 
good  membranes  and  in  maintaining  them  in  effective  condition  there- 
after is  due  to  the  mischievous  influence  of  potassium  upon  the  colloidal 
state  of  the  membrane  material  led  to  several  modifications  of  the 
method  of  building  up  membranes.  The  first  of  these  was  a  consid- 
erable dilution  of  the  solutions  which  are  used  in  forming  them,  a 
dilution  from  0.1  to  0.01  weight-ion-normal.  Somewhat  later  the  con- 
centration of  the  "membranogens, "  which  are  used  within  and  without 
the  cells  while  measuring  pressure,  was  reduced  from  0.01  to  0.001 
normal.  The  effects  were  seemingly  good,  in  that  higher  resistances 
could  be  obtained  at  any  single  " running"  than  before  and  the  cells 
appeared  to  require  less  soaking  between  measurements.  Still  later  a 
0.01  weight-ion-normal  solution  of  ferrocyanic  acid  was  substituted  for 
the  potassium  salt  in  all  operations  connected  with  the  deposition  and 
reinforcement  of  membranes.  The  results  of  the  last  substitution  are 
very  promising,  and  the  acid  seems  likely  wholly  to  displace  the  salt. 

It  has  been  found  that,  while  potassium  salts  are  in  a  marked  degree 
injurious  to  the  membranes  and  may  easily  ruin  them,  the  salts  of 
lithium  are  comparatively  harmless.  This  discovery  has  been  utilized 
to  some  extent  and  with  advantage  in  the  deposition  of  membranes  by 
substituting  the  ferrocyanide  of  lithium  for  that  of  potassium. 

4.  ACTIVITY  OF  THE  MEMBRANE. 

In  discussing  the  so-called  "thermometer  effects,"  which  are  due  to 
fluctuations  of  temperature,  the  fact  was  emphasized  that  the  passage 
of  solvent  through  the  membrane  is  by  no  means  instantaneous,  and 
that  it  may  be  exceedingly  slow.  We  have,  therefore,  a  "barometer 
effect''  due  to  fluctuations  of  atmospheric  pressure.  Barometer  effects 
are,  however,  less  troublesome  than  thermometer  effects,  because,  as 
a  source  of  error  in  the  measurement  of  osmotic  pressure,  their  mag- 
nitude is  limited  to  the  comparatively  small  variations  in  atmospheric 
pressure.  Moreover,  the  errors  due  to  them  can  be  eliminated  by 
correcting  the  mean  of  all  the  daily  observations  of  osmotic  pressure 
by  the  mean  barometric  pressure  for  the  whole  period  within  which 
an  experiment  is  in  progress. 

The  activity  of  a  membrane,  i.  e.,  the  rate  at  which  the  solvent  will 
pass  through  it,  depends,  of  course,  in  the  first  place,  upon  its  area. 
Since,  however,  the  membrane  is  made  up  of  a  multitude  of  little 
"  plugs,"  which  fill  the  mouths  of  the  pores  opening  upon  the  interior 
of  the  cell,  the  area  of  the  membrane  is  equal  to  the  aggregate  area 
of  these  pores  at  their  mouths.  In  other  words,  the  size  of  the  mem- 
brane depends,  not  on  the  area  of  the  interior  of  the  cell,  but  upon  the 
number  and  the  size  of  the  pores  in  the  cell  wall.  It  is  this  fact  which 
makes  all  quantitative  comparisons  of  the  membranes  of  different  cells 
impossible.  We  can  never  know  their  relative  areas.  It  is  only  known 
that,  other  things  being  probably  equal,  water  passes  more  slowly 


THE   MEMBRANES. 


89 


through  a  membrane  in  a  hard-burned  cell,  in  which  the  pores  are 
presumably  small,  than  it  does  through  the  membrane  of  a  soft-burned 
one,  in  which  the  pores  are  presumably  large. 

The  rate  at  which  water  will  pass  through  a  given  membrane  de- 
creases with  age  and  use.  In  cells  with  new  membranes,  the  osmotic 
pressures  of  solutions  often  attain  a  maximum  within  six  hours,  pro- 
vided the  temperatures,  and  therefore  the  volumes  of  the  solutions, 
remain  constant;  while  the  same  cells,  two  years  later,  may  require 
from  five  to  ten  days,  or  even  longer,  for  the  establishment  of  equi- 
librium pressures.  In  Table  4  the  records  of  the  determinations  of 
osmotic  pressure  illustrate  the  difference,  as  regards  the  time  required  for 
the  development  of  equilibrium,  between  an  excellent  new  membrane 
and  an  old  one,  which,  though  slow,  is  otherwise  in  good  condition  for 
the  measurement  of  osmotic  pressure : 

TABLE  4. — Observed  osmotic  pressures. 


I.  New  membrane. 
0.9  weight-normal  solution  of 
cane  sugar.     Temperature  25°. 


First  day 24.127 

Second  day 24. 148 

Third  day 24.125 

Fourth  day 24.102 

Fifth  day 24.125 


Mean..  24.126 


II.  Old  membrane. 
0.6  weight-normal  solution  of 
cane  sugar.     Temperature  25°. 


Sixth  day 15.654 

Seventh  day 15 . 612 

Eighth  day 15.628 

Ninth  day 15.629 


Mean..  15.627 


The  maximum  pressure  was  reached  in  less  than  six  hours  in  the 
case  of  the  new  membrane  cited  above,  while  six  full  days  were  required 
in  that  of  the  old  one.  In  another  instance,  the  maximum  pressure 
was  reached  on  the  tenth  day  and  it  remained  constant  for  12  days, 
when  the  cell  was  opened.  The  measurement  was  an  excellent  one, 
and  the  only  defect  of  the  cell  was  the  excessive  slowness  with  which 
the  solvent  passed  through  the  membrane  into  the  solution.  It  should 
also  be  noted  in  this  place  that  certain  cells  whose  membranes  had 
suffered  some  deterioration  through  contact  with  electrolytes  have  been 
known  to  require  more  than  20  days  for  the  establishment  of  final 
pressures.  The  measurements  were,  nevertheless,  entirely  satisfactory. 

In  cells  with  old  membranes,  and  in  those  whose  membranes  have 
become  slow  through  contact  with  electrolytes,  "thermometer"  and 
"barometer"  effects  are  necessarily  large;  hence  when  small  pressures, 
i.  e.,  those  of  dilute  solutions,  are  to  be  measured,  cells  with  young 
and  especially  active  membranes  are  selected.  For  such  purposes, 
soft-burned  cells  have  the  obvious  advantage  that  in  them  the  areas 
of  the  membranes  are  relatively  large.  If  the  pressures  to  be  measured 
are  minute,  they  may  be  entirely  masked  by  the  thermometer  and 
barometer  effects.  To  illustrate  this  point,  it  is  recalled  that  certain 


90        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

small  amounts  of  the  membrane-forming  compounds  are  employed 
in  measuring  osmotic  pressure  for  the  purpose  of  mending  any  rents 
which  may  be  made  in  the  membranes.  The  compounds,  in  the  quanti- 
ties used,  are  supposed  to  be  osmotically  equivalent,  and  therefore 
to  have  no  effect  upon  the  observed  osmotic  pressures;  but  this  can  not 
be  proved,  because  the  difference,  if  any,  is  less  than  the  unavoidable 
thermometer  and  barometer  effects.  Similar  difficulties  are  encoun- 
tered when  it  is  attempted  to  employ  very  small  membranes.  On 
certain  occasions,  in  the  course  of  the  present  investigation,  it  was 
desirable  to  employ  membranes  of  very  limited  area.  To  obtain  these, 
the  cells  were  glazed  over  the  whole  surface,  interior  and  exterior,  and 
afterwards  ground  off  on  opposite  sides  of  the  cell  over  as  much  of  the 
"biscuit"  as  it  was  desired  to  expose  for  the  membrane.  Such  cells 
were  found,  however,  to  be  quite  impracticable,  because  of  their  slow- 
ness in  responding  to  fluctuations  of  bath  temperature  and  atmospheric 
pressure.  Since,  other  things  being  equal,  the  time  required  for  the 
establishment  of  equilibrium  pressure  and  the  magnitude  of  the  ther- 
mometer and  barometer  effects  are  inversely  proportional  to  the  area 
of  the  membrane,  it  is  obviously  desirable  to  make  the  membranes  as 
large  as  they  may  be  consistently  with  other  requirements. 

"Slow  cells"  are  kept  under  observation  for  much  longer  periods 
than  "quick"  ones,  because  of  the  minimizing  effect  of  time  on  the 
magnitude  of  thermometer  and  barometer  effects. 

After  hundreds  of  quantitative  measurements  of  osmotic  pressure, 
we  are  still  unable  to  say  how  the  rate  of  passage  of  the  solvent  through 
the  membrane  is  affected  by  the  concentration  of  the  solution  within 
the  cell.  The  difficulty  in  settling  a  question  of  this  kind  is  due  to 
the  fact  that  no  two  membranes  are  exactly  alike  in  all  the  elements 
which  determine  the  rate  of  transference.  It  has  already  been  stated 
that  the  membranes  of  no  two  cells  are  of  equal  area;  and  it  is  also  true 
that  the  membrane  in  any  given  cell  is  never  exactly  the  same  on  two 
successive  occasions.  The  effect  of  temperature  upon  the  activity  of  the 
membranes  is  also  uncertain,  though  the  writer  and  his  associates  are 
under  the  impression  that  a  rise  in  temperature  increases  their  activity. 
But  here  again  quantitative  comparisons  are  impossible. 

If  a  cell  is  set  up  with  a  solution,  under  a  small  initial  mechanical 
pressure,  it  is  noticed  that  the  rise  of  the  mercury  in  the  manometer 
is  very  rapid  at  first,  but  that  the  rate  of  ascent  decreases  with  great 
regularity  and  becomes  exceedingly  slow  as  the  meniscus  approaches 
its  final  position.  This  appears  to  indicate  that  the  rate  of  passage 
of  the  solvent  through  the  membrane  depends  on  the  difference  between 
the  pressure  existing  in  the  cell  and  the  true  osmotic  pressure  of  the 
solution.  There  are,  however,  no  means  of  determining  how  the  whole 
time  required  for  the  establishment  of  equilibrium  is  related  either  to 
the  concentration  or  to  the  temperature  of  the  solution. 


THE    MEMBRANES.  91 

5.  DETERIORATION  OF  THE  MEMBRANE. 

If  the  ferrocyanide  of  zinc  (the  membrane  of  Tamman)  is  deposited 
in  a  cell  in  the  usual  manner  and  is  tested  soon  thereafter  with  a 
solution  of  sugar,  considerable  pressure  is  developed  on  the  first  trial, 
but  by  no  means  the  full  osmotic  pressure  of  the  solution.  Moreover, 
the  pressure  does  not  at  any  time  become  constant,  but,  having  reached 
its  highest  development,  it  falls  slowly  and  continuously  until  it  is  in 
equilibrium  with  the  pressure  of  the  air.  If  the  cell  is  now  emptied 
and  soaked  in  water,  and  the  membrane  is  reinforced  in  the  usual 
manner,  and  the  cell  is  again  set  up  with  a  solution  of  sugar,  some 
pressure  is  developed,  but  always  less  than  on  the  first  trial.  On  each 
succeeding  trial,  a  still  smaller  pressure  is  obtained,  until  at  last  the 
cell  develops  no  pressure  whatever.  After  the  first  trial,  the  solutions 
which  are  removed  from  the  cell  have  a  milky  appearance,  due  to 
suspended  ferrocyanide  of  zinc;  and  on  close  examination  the  com- 
pound is  found  to  have  lost  its  original  structureless  (colloidal)  con- 
dition and  to  have  become  granular,  though  not  distinctly  crystalline. 
We  have  here  a  clear  case  of  degeneration  which  appears  to  consist  in 
a  change  in  the  membrane  material  from  a  gelatinous  or  colloidal 
condition  to  a  granular  state.  From  the  fact  that  after  a  time  no 
pressure  can  be  obtained,  however  much  the  membrane  may  be  soaked 
in  water  or  reinforced  by  the  deposition  of  additional  material,  it  is 
inferred  that  during  the  later  experiments  the  newly  formed  ferro- 
cyanide is  transformed  as  fast  as  it  is  deposited.  A  similar,  but  less 
striking,  degeneration  has  been  noticed  on  the  part  of  the  manganese 
ferrocyanide  membrane.  Neither  the  zinc  nor  the  manganese  salt  has 
been  found  suitable  for  the  measurement  of  osmotic  pressure. 

The  ferrocyanide  of  copper  membrane,  when  carefully  treated  in 
the  prescribed  manner,  appears  to  suffer  no  such  deterioration  as  long 
as  it  is  used  to  measure  the  pressure  of  non-electrolytes  only,  and  at 
moderate  temperatures.  It  is  still  uncertain  whether  the  disastrous 
effect  of  rapid  cooling,  after  using  the  membrane  at  high  temperature, 
is  due  to  a  similar  transformation,  or  not.  The  membrane  becomes 
less  active  with  age,  but  not  ineffective.  In  fact,  old  membranes  are 
preferred  for  the  measurement  of  high  pressures,  because  of  their  great 
strength  and  reliability;  and  old  cells  are  discarded  only  when  the 
passage  of  solvent  through  their  membranes  becomes  intolerably  slow, 
and  never  because  they  will  not  measure  correctly,  if  given  time  enough. 
Some  cells  have  been  in  use  more  than  four  years.  The  decreased 
activity  of  old  membranes  may  be  due,  in  part  at  least,  to  the  thickening 
effect  of  the  frequent  reinforcement  with  new  material  to  which  they 
are  subjected. 

The  conduct  of  the  ferrocyanides  of  nickel  and  cobalt  resembles  that 
of  the  ferrocyanide  of  copper.  Both  of  them  give  membranes  which 
do  not  appear  to  degenerate  under  the  influence  of  non-electrolytes. 


92       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

The  same  is  probably  true  of  a  number  of  the  cobalticyanides,  but  the 
experimental  evidence  with  regard  to  the  last  class  of  salts  is  still  too 
meager  to  warrant  any  positive  statements  concerning  their  durability. 
A  ferrocyanide  of  copper  membrane  can  be  satisfactorily  reinforced 
with  the  nickel  or  the  cobalt  salt,  and  vice  versa. 

6.  THE  EFFECT  OF  ELECTROLYTES. 

It  has  been  shown  that  the  effect  on  the  membranes  of  the  alkali 
which  is  liberated  during  their  deposition  is  undoubtedly  injurious; 
and  the  impossibility  of  building  up  good  membranes  and  of  main- 
taining them  thereafter  in  good  condition,  without  repeated  and  pro- 
longed soaking  in  pure  water,  has  been  ascribed  to  the  accumulation 
of  potassium  in  the  membranes.  It  was  therefore  apprehended  that  the 
measurement  of  the  osmotic  pressure  of  potassium  salts,  and  perhaps 
of  other  electrolytes,  would  be  attended  with  great  difficulties.  These 
fears  have  been  partially,  but  not  fully,  realized.  Our  experience  with 
the  electrolytes  will  be  given  in  Chapter  XI,  which  describes  our  efforts 
to  measure  the  osmotic  pressures  of  potassium  and  lithium  salts. 

7.  THE  SEMIPERMEABILITY  OF  MEMBRANES. 

It  is  often  asserted  that  no  membrane  is  truly  semipermeable;  in 
other  words,  it  is  frequently  affirmed  that  all  membranes  are  perme- 
able to  the  solute  as  well  as  to  the  solvent — that  the  difference  is  one 
of  degree  only.  Such  statements  appear  to  be  without  justification. 
They  are  certainly  not  founded  on  any  reliable  information  in  the 
possession  of  those  who  make  them.  The  question  is  one  of  funda- 
mental importance  and  is  to  be  decided  only  by  experiment.  The 
fact  that  all  the  solutions  of  cane  sugar  and  glucose  which  are  taken 
from  good  cells,  after  a  measurement  of  osmotic  pressure,  are  found 
to  have  maintained  their  concentration  perfectly,  is  evidence  enough 
that  membranes  may  be  made  sufficiently  semipermeable  for  quanti- 
tative purposes.  But,  though  many  of  our  cells  had  maintained,  with- 
out evidence  of  weakness,  the  maximum  pressures  of  the  solutions  for 
10,  15,  and  even  20  days,  it  was  decided  to  test  the  soundness  of  the 
membranes  by  means  of  an  experiment  of  much  longer  duration.  For 
this  purpose,  a  cell  containing  a  0.5  weight-normal  solution  of  cane 
sugar  was  selected  at  random  and  allowed  to  remain  in  the  bath  at 
15°  for  two  full  months.  The  record  of  the  cell  is  given  in  Table  5  in 
atmospheres  of  osmotic  pressure. 

At  the  end  of  the  two  months,  the  solution  was  removed  from  the 
cell  and  compared  in  the  polariscope  with  a  reserved  portion  of  the 
original  solution.  The  rotations  of  the  two  were  identical,  showing 
that  no  leakage  of  the  solute  had  occurred. 

The  cell  employed  in  this  experiment  is  a  good  example  of  what 
came  to  be  known  as  "quick  cells."  In  less  than  24  hours  after  setting 


THE   MEMBRANES. 


93 


it  up,  the  osmotic  pressure  which  it  registered  was  12.522  atmospheres, 
the  barometric  pressure  being  1.013.  On  the  sixtieth  day  thereafter, 
the  osmotic  pressure  was  also  12.522  atmospheres  and  the  barometric 
pressure  was  1.016.  The  lowest  osmotic  pressure  observed  during  the 
two  months  was  12.517;  the  highest  was  12.552.  The  extreme  (appar- 
ent) fluctuation  in  osmotic  pressure  was  therefore  0.035  atmosphere, 
or  0.28  per  cent  of  the  mean  (12.533  atmospheres)  of  all  observations. 
The  extreme  fluctuation  in  atmospheric  pressure  during  the  same  period 
was  0.035  atmosphere.  In  other  words,  the  extremes  of  variation  were 
the  same  for  osmotic  and  barometric  pressures.  The  highest  (apparent) 
osmotic  pressures  were  contemporaneous  throughout  with  the  lowest 
atmospheric  pressures,  and  vice  versa.  This  is  due,  of  course,  to  the 
fact  that,  owing  to  the  slowness  with  which  the  solvent  passes  through 
the  membrane,  the  pressure  within  the  cell  can  not  immediately  adjust 
itself  to  changes  in  atmospheric  pressure. 

TABLE  5. — Observed  osmotic  pressures. 


Day. 

Atmos- 
pheres. 

Day. 

Atmos- 
pheres. 

Day. 

Atmos- 
pheres. 

Day. 

Atmos- 
pheres. 

2d 

12.522 

17th. 

12  .  539 

32d.. 

12.537 

47th. 

12.534 

3d 

12.535 

18th. 

12.533 

33d.. 

12.531 

48th. 

12.544 

4th  . 

12.544 

19th. 

12.527 

34th. 

12.536 

49th. 

12.533 

5th. 

12.534 

20th. 

12.530 

35th. 

12.533 

50th. 

12.532 

6th. 

12.536 

21st.  . 

12.527 

36th. 

12.532 

51st. 

12.552 

7th. 

12.552 

22d.. 

12.536 

37th. 

12.537 

52d.. 

12.533 

8th. 

12.535 

23d.. 

12.525 

38th. 

12.517 

53d.. 

12.532 

9th. 

12.536 

24th. 

12.526 

39th. 

12.517 

54th. 

12.535 

10th. 

12.538 

25th. 

12.536 

40th. 

12.524 

55th. 

12.533 

llth. 

12.524 

26th. 

12.534 

41st. 

12.529 

56th. 

12.540 

12th. 

12.541 

27th. 

12.524 

42d.. 

12.533 

57th. 

12.536 

13th. 

12.537 

28th. 

12.537 

43d.. 

12.523 

58th. 

12.545 

14th. 

12.529 

29th. 

12.524 

44th. 

12.528 

59th. 

12.539 

15th. 

12.532 

30th. 

12.535 

45th. 

12.530 

60th. 

12.527 

16th. 

12.531 

31st.  . 

12.535 

46th. 

12.546 

61st. 

12.522 

8.  REMOVAL  OF  THE  MEMBRANE. 

When,  through  age  and  frequent  reinforcement,  or  through  the  dete- 
rioration due  to  contact  with  electrolytes,  a  membrane  has  become 
intolerably  "slow"  or  otherwise  unserviceable,  it  is  desirable  to  replace 
it  by  a  new  one.  According  to  Pfeffer,*  an  old  membrane  of  copper 
ferrocyanide  can  be  removed  and  successfully  replaced  by  a  new  one. 
For  the  removal,  he  recommends  soaking  the  cell  in  a  dilute  solution 
of  potassium  hydroxide,  to  which  has  been  added  a  little  Rochelle  salt, 
and  afterwards  in  water,  in  hydrochloric  acid,  and  again  in  water. 
The  removal  of  the  membrane  by  this  method  is  easy,  but  we  have 
never  been  able,  after  such  treatment  of  a  cell,  to  build  up  in  it  a 
thoroughly  good  new  membrane.  The  process  has  been  modified  in 


*"Osmotische  Untersuchungen,"  12. 


94  OSMOTIC   PRESSURE    OF  AQUEOUS   SOLUTIONS. 

various  ways,  but  without  entirely  satisfactory  results.  One  of  the 
modifications  consisted  in  the  omission  of  both  the  caustic  potash  and 
the  hydrochloric  acid,  and  in  the  removal,  by  electrolysis,  of  the  soluble 
salts  which  were  left  in  the  wall  after  soaking  the  cell  in  water.  The 
best  results  were  obtained  by  a  simple  electrolysis  of  the  membranes 
in  the  presence  of  water,  but  the  removal  of  membranes  by  this  method 
is  exceedingly  slow.  Fairly  good  results  were  also  obtained  by  grind- 
ing off  the  interior  wall  of  the  cells  with  a  carborundum  wheel  running 
at  high  speed,  and  afterwards  reburning  them.  In  whatever  manner 
the  membranes  may  be  removed,  the  reburning  is  essential.  It  is 
suspected  that  the  walls  of  the  pores,  near  their  mouths,  are  in  some 
way  modified  (perhaps  made  more  smooth)  by  the  reagents  which  are 
employed  to  remove  the  membranes,  with  the  result  that  the  "plugs" 
of  the  new  membranes  do  not  fit  so  firmly  into  their  places.  It  is  now 
preferred  to  discard  cells  with  old  or  defective  membranes  rather  than 
attempt  to  restore  them  to  use  by  replacing  their  membranes. 

9.  INFECTION  OF  THE  MEMBRANES. 

The  ready  infection  of  the  membranes  by  voracious  nitrogen-con- 
suming fungi  has  been  one  of  the  serious  obstacles  to  the  progress  of 
the  present  investigation.  The  particular  fungus  known  to  have  pro- 
duced a  large  amount  of  mischief  is  a  strain  of  Penicillium  glaucum. 
Others  are  believed  to  have  contributed  to  the  frequent  destruction 
of  the  membranes,  but  they  have  not  been  identified  with  certainty. 

The  first  announcement*  regarding  this  pest  was  as  follows: 

"Soon  after  beginning  the  measurement  of  osmotic  pressure,  there  appeared 
upon  one  of  our  cells  an  abundant  growth  of  a  fungus,  which  upon  examination 
was  found  to  be  penicillium.  Within  the  next  few  days,  it  appeared  upon  one 
after  another  of  the  remaining  cells,  until  all  were  affected  in  the  same  manner 
as  the  first.  We  then  exposed  several  solutions  of  glucose  to  the  air  of  the 
laboratory,  and  the  fungus  appeared  in  all  of  them  in  a  short  time.  We  had 
had  no  similar  experience  previously,  though  we  had  worked  with  solutions 
containing  invert  sugar  more  than  two  years,  and  the  conditions  under  which 
we  were  working  were  in  general  unfavorable  to  the  fungus.  The  sudden 
prevalence  of  penicillium  spores  in  the  atmosphere  of  the  laboratory  could, 
however,  be  accounted  for,  though  it  had  not  been  anticipated.  At  the  time, 
certain  changes  were  in  progress  in  the  lower  part  of  the  building  which 
involved  the  tearing  away  of  old  walls,  and  the  atmosphere  of  all  parts  of  the 
laboratory  was,  in  consequence,  in  a  somewhat  dusty  condition." 

A  search  was  immediately  instituted  for  some  poison  which  would 
kill  the  fungus  without  injuring  the  membranes.  Of  the  numerous 
substances  which  were  tested,  hydrocyanic  acid  in  gaseous  form  and 
thymol  were  found  to  be  quite  effective  and  entirely  harmless  to  the 
membranes.  Formaldehyde  has  also  been  extensively  employed,  espe- 
cially for  the  disinfection  of  the  baths. 

*Amer.  Chem.  Journal,  xxxvi,  34. 


THE   MEMBRANES.  95 

The  first  intimation  of  infection  is,  of  course,  the  fact  that  the  cells 
will  not  develop  maximum  pressures.  On  removing  the  solutions,  they 
are  found  always  to  have  a  more  or  less  greenish  color,  and  the  absence 
of  this  color  is  a  sure  sign  that  the  membrane  has  not  been  attacked 
by  the  fungus.  If  the  destruction  of  the  membrane  by  the  penicillium  is 
far  advanced,  there  are  also  found,  upon  the  bottom  of  the  cell,  minute 
grains  of  a  substance  whose  color  varies  from  green  to  blue.  When  the 
fungus  is  allowed  to  grow  in  a  solution  of  sugar  to  which  some  membrane 
material  (copper  ferrocyanide)  has  been  added,  the  solution  becomes 
green,  then  blue,  and  finally  brown  from  suspended  iron  hydroxide,  as 
if  the  whole  of  the  nitrogen  of  the  ferrocyanide  had  been  appropriated. 

The  restoration  to  a  usable  condition  of  a  membrane  which  has  been 
attacked  by  this  penicillium  is  a  work  of  some  months.  The  quickest 
method  of  killing  the  fungus  is  to  place  the  wet  cell  under  a  bell  jar  and  to 
develop  in  the  inclosed  space  gaseous  hydrocyanic  acid  by  dripping  dilute 
hydrochloric  acid  into  a  dish  containing  potassium  cyanide.  Though  it 
grows  vigorously  in  dilute  solutions  of  copper  sulphate  which  are  exposed 
to  the  air,  it  soon  dies  in  a  saturated  solution  of  thymol.  It  is  quickly 
killed  by  formaldehyde  in  dilute  solution.  Phenol  and  salicylic  acid 
appear  to  be  less  poisonous  to  it  than  thymol. 

After  destroying  the  penicillium,  it  is  necessary  to  begin  anew  and 
to  repeat  in  every  detail  the  series  of  operations  employed  in  building 
up  and  seasoning  membranes. 

Because  of  the  laborious  character  of  the  measures  which  must  be 
resorted  to  for  the  restoration  of  the  membranes,  every  possible  pre- 
caution is  taken  to  prevent  infection.  Some  of  these  preventives  have 
already  been  mentioned  incidentally — for  example,  the  boiling  of  all 
water  which  comes  in  contact  with  the  cells,  the  careful  covering  of  all 
vessels,  the  soaking  of  the  cells  in  thymol  water,  and  the  addition  of 
minute  quantities  of  thymol  to  the  liquids  within  and  without  the  cell 
when  a  measurement  of  pressure  is  to  be  made.  Another  precaution 
consists  in  the  occasional  disinfection  of  the  baths  at  high  temperature 
(from  70°  to  80°)  with  the  vapors  of  formaldehyde,  which  are  circu- 
lated within  the  inclosed  spaces  by  means  of  fans.  The  interior  walls 
of  the  baths,  together  with  all  their  fittings,  such  as  wires,  etc.,  are 
frequently  washed  with  a  solution  of  formaldehyde.  In  the  older  form 
of  "rectangular"  bath  the  water  and  air  spaces  were  not  carefully 
separated.  The  upper  or  air  space  was  therefore  always  saturated 
for  the  given  temperature  with  water  vapor,  producing  a  condition 
which  was  especially  favorable  to  the  growth  of  the  penicillium.  It 
was  found  impossible  to  rid  these  baths  of  infection  for  any  length  of 
time.  They  were,  therefore,  all  reconstructed  with  vapor-tight  par- 
titions between  the  two  compartments.  It  was  afterwards  practicable 
to  keep  the  air  spaces  so  dry  (by  means  of  desiccating  agents)  that  the 
fungus  could  not  grow. 


CHAPTER  V. 
THE  WEIGHT-NORMAL  SYSTEM  FOR  SOLUTIONS. 

The  solutions  employed  in  this  investigation  have  been  made,  from 
the  beginning,  by  dissolving  a  gram-molecular  weight  of  the  substance, 
or  a  decimal  part  of  the  same,  in  1,000  grams  of  water.  Moreover,  in 
calculating  the  theoretical  gas  pressure  of  the  solute  at  any  temperature 
the  volume  of  the  solvent  in  the  pure  state,  and  not  that  of  the  solution, 
has  been  adopted  as  the  standard.  The  solutions  so  made  have  been 
called  "weight-normal"  to  distinguish  them  from  " volume-normal" 
solutions,  or  those  made  by  dissolving  the  same  quantities  of  substance 
and  diluting  the  solutions  to  a  volume  of  1,000  cubic  centimeters. 
Perhaps  a  better  name  for  such  solutions  would  have  been  "solvent- 
normal." 

There  have  existed  a  widespread  and  persistent  misapprehension  of 
the  reasons  which  led  to  the  adoption  of  the  weight-normal  system  for 
osmotic-pressure  measurements  and  a  quite  general  misunderstanding 
of  the  nature  of  the  advantages  which  were  expected  from  its  employ- 
ment. Unfortunately,  but  perhaps  not  altogether  unnaturally,  it  has 
been  inferred  by  many  that  the  preference  shown  for  this  system  has 
somehow  committed  the  author  and  his  associates  to  the  view  that 
under  it  the  osmotic  pressure  of  aqueous  solutions  will  be  found  to 
conform  necessarily  to  the  gas  laws.  This  appears  to  be  the  interpreta- 
tion of  Findlay,  who,  in  his  recent  excellent  work*  on  osmotic  pressure, 
has  reduced  the  weight-normal  system,  as  employed  by  the  writer 
and  his  co-workers,  to  the  form  of  a  general  equation  which  he  calls  the 
" equation  of  Morse,"  and  has  then  proceeded  to  show — though  by 
evidence  which  is  not  entirely  convincing  to  the  writer — that  it  must 
fail  in  the  case  of  highly  concentrated  solutions.  It  is  hoped  that  the 
following  somewhat  discursive  presentation  of  the  subject  will  serve  to 
clear  up  some  of  the  misunderstandings  which  have  arisen. 

The  fact  that  van't  Hoff  expressly  limited  his  deductions  concerning 
osmotic  pressure  to  extremely  dilute  solutions,  in  which  neither  the 
aggregate  volume  of  the  solute  molecules  nor  their  mutual  attractions 
are  of  moment,  has  been — certainly  until  recently — too  often  ignored 
or  too  feebly  emphasized.  That  he  had  a  clear  vision  of  some  of  the 
complications  which  must  arise  when  it  was  attempted  to  deal  with 
the  osmotic  pressure  of  concentrated  solutions,  and  that  he,  on  that 
account,  deliberately  excluded  these  as  something  for  which  the  simple 
equation  PV  =  KT  is  inadequate,  is  convincingly  shown  by  the  follow- 

*"  Osmotic  Pressure,"  by  Alexander  Findlay.     Longmans,  Green  &  Co.,  1913. 

97 


98       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

ing  quotations  from  van't  Hoff's  memorable  paper.*  At  the  conclu- 
sion of  Section  I,  entitled  "Der  Osmotische  Druck,  Art  der  Analogie, 
welche  durch  dessen  Einfuhrung  entsteht,"  he  says  (page  483) : 

"Von  diesem  praktischen  Vorteil  werden  wir  in  Nachfolgenden  Nutzen 
ziehen,  speziell  zur  Erforschung  der  fur  ideale  Losungen  giltigen  Gesetze,fur 
Losungen  also,  die  derartig  verdiinnt  sind,  dass  sie  den  idealen  Gasen  an  die 
Seite  zu  stellen  sind,  und  in  denen  somit  die  gegenseitige  Wirkung  der  gelosten 
Molekule  zu  vernachldssigen  ist,  wie  auch  der  von  diesen  Molekillen  eingenom- 
mene  Raum  bei  Vergleich  mit  dem  Volum  der  Losung  selbst." 

The  succeeding  three  sections,  namely,  II,  III,  and  IV,  are  entitled: 

II.  "Boyle's  Gesetz/wr  verdunnte  Losungen." 

III.  "Gay-Lussac's  Gesetz/wr  verdunnte  Losungen." 

IV.  "Avogadro's  Gesetz/wr  verdunnte  Losungen." 

Again,  he  says  (page  498) : 

"Noch  trefflicher  ist  dass  der  so  allgemein  auch  fiir  Losungen  angenom- 
mene  Guldberg  und  Waagesche  Satz  thatsachlich  als  einfache  Schlussfolgerung 
aus  den  oben  fur  verdunnte  Losungen  aufgestellten  Gesetzen  entwickelt  werden 
kann." 

It  is  equally  certain  that  some  of  those  who  were  foremost  in  adopting 
the  new  views  concerning  osmotic  pressure,  and  who  became  the  most 
effective  agents  in  promoting  their  publicity  and  general  acceptance, 
failed  to  make  clear  the  full  significance  of  van't  Hoff's  reservations; 
for  we  find  stated,  without  due  qualification,  and  thereafter  constantly 
repeated  as  models  of  concise  yet  comprehensive  definition,  proposi- 
tions like  the  following: 

"  Dissolved  substances  exert  the  same  pressure,  in  the  form  of  osmotic  pressure, 
as  they  would  exert  were  they  gasified  at  the  same  temperature  without  change  of 
volume."  [Again]  *  *  *  "d.  h.  der  osmotische  Druck  gelosten  Rohrzucker  ist 
gerade  so  gross  wie  der  Gasdruck  den  man  beobachten  wiirde,  wenn  man  das 
Losungsmittel  entfernte,  und  die  geloste  Substanz  den  gleichen  Raume  bei  gleicher 
Temper atur  in  Gasform  erfullend  zuruckliesse." 

Much  confusion  and  futile  discussion  would  have  been  saved  if  it 
had  been  clearly  explained  in  connection  with  all  such  statements: 

1.  That  van't  Hoff  intended  to  apply  the  equation  PV  =  KT  only 
to  "ideal  solutions,"  i.  e.,  to  solutions  so  dilute  that  neither  the  volume 
nor  the  mutual  attractions  of  the  solute  molecules  are  of  importance. 

2.  That  when  more  concentrated  solutions  are  to  be  dealt  with,  it 
will  obviously  be  necessary  to  modify  the  simple  equation  for  "ideal" 
solutions  in  a  manner  analogous  to  the  modification  by  van  der 
Waals  of  the  equation  for  "ideal  gases." 

3.  That,  because  of  the  solvent,  the  case  of  solutions  is  more  complex 
than  that  of  gases,  and  that,  for  this  reason,  the  general  equation  for 
them  may  be  more  complex  than  the  equation  of  van  der  Waals  for 
gases. 

*Zeitschrif  t  fur  physikalische  Chemie,  I,  479. 


WEIGHT-NORMAL   SYSTEM   FOR   SOLUTIONS.  99 

Apparently  the  confusion  of  mind  which  prevailed  for  several  years 
after  the  publication  of  van't  Hoff's  paper  —  and  of  which  one  sees 
many  evidences,  even  at  the  present  time  —  was  due,  in  great  part, 
to  the  persistence  of  the  habit  of  regarding  the  simple  equations  which 
apply  to  so-called  "ideal"  conditions  as  the  embodiments  of  the  general 
laws  for  gases  and  solutions.  The  really  comprehensive  equation  for 


is,  of  course,  that  of  van  der  Waals    -^+2    (V  —  b)=RT;  while 


uation  for  so-called  "ideal"  gases,  PV  =  RT,  covers  only  a  special, 
and,  In  fact,  a  purely  imaginary  and  impossible  case,  that,  namely, 

in  which  the  ™  and  the  b  of  van  der  Waal's  equation  have  become  zero. 

It  is  conceivable  that  in  the  course  of  time  an  approximately  com- 
prehensive equation  will  be  developed  for  osmotic  pressure,  but,  in  the 
opinion  of  the  writer,  it  will  be  the  fruit  of  extensive  and  painstak- 
ing experimental  research  rather  than  of  ingenious  speculation.  In 
other  words,  it  will  be  the  embodiment  of  the  general  rule  which  is 
finally  formulated  for  the  purpose  of  correlating  a  great  variety  of 
authenticated  facts  concerning  osmotic  pressure.  The  equation  of 
van't  Hoff  will,  of  course,  stand  in  much  the  same  relation  to  it  as 
does  the  expression  PV  =  RT  to  the  more  general  equation  of  van  der 
Waals.  It  is  doubtful,  however,  if  any  proposed  general  equation  for 
osmotic  pressure,  although  containing  suitable  terms  for  all  the  factors 
which  must  be  taken  into  account,  would  be  of  any  present  utility  in 
the  case  of  aqueous  solutions,  since  the  value  of  at  least  some  of  these 
terms — e.  g.,  that  covering  hydration — must  still  be  experimentally 
determined  for  every  solute  and  at  every  temperature  and  in  each 
individual  concentration  of  solution.  If  it  is  true  that  the  value  of  an 
equation  is  to  be  measured  by  its  competence  to  foretell  the  truth  in 
any  case  to  which  it  may  appropriately  be  applied,  then  every  general 
equation  for  osmotic  pressure  is  bound  to  disappoint  one  who  attempts 
to  apply  it  to  aqueous  solutions.  To  illustrate :  There  is  no  equation 
conceivable  which  could  foretell,  in  the  case  of  cane-sugar  solutions, 
that  the  value  of  the  hydration  term  is  constant  for  each  concentra- 
tion between  0°  and  25°,  or  that  it  soon  thereafter  begins  to  decline 
in  value  to  become  zero  at  some  definite  higher  temperature;  or 
further,  how  what  may  be  called  the  idiosyncrasies  of  hydration  may 
be  expected  to  vary  from  one  solute  to  another.  In  view  of  the 
necessary  limitations  of  its  usefulness  as  a  means  of  discovering  truth, 
the  author  does  not  regard  a  general  equation  as  the  ultimum  bonum  in 
the  field  of  osmotic  pressure  or  (in  the  present  meager  and  inexact 
state  of  our  knowledge  of  the  subject)  as  even  highly  desirable.  The 
osmotic  pressure  of  solutions — especially  of  aqueous  solutions — depends 
upon  such  a  variety  of  still  unmeasured  and  imperfectly  understood 
conditions  that  any  attempted  comprehensive  expression  for  it  at  the 


100  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 

present  time  must  fail  to  convince,  and  is  bound  to  absorb  in  futile 
discussion  much  energy  which  might  be  more  profitably  employed  in 
finding  out  what  the  facts  of  osmotic  pressure  really  are. 

It  has  been  intimated  by  some  of  our  friendly  colleagues  that  the 
adoption  at  the  beginning  of  our  investigation  (1)  of  the  weight-normal 
system  for  the  solutions;  (2)  of  the  practice  of  referring  the  gas  pressure 
of  the  solute  to  the  volume  of  the  solvent;  and  (3)  of  the  custom  of 
always  stating  the  ratio  of  observed  osmotic  pressure  to  the  calculated 
gas  pressure  of  the  solute  are  all  inconsistent  with  the  general  attitude 
toward  the  subject  which  is  professed  above — that,  in  fact,  all  three 
of  the  itemized  practices  are  indicative  of  preformed  judgments  in  a 
case  which  we  were  professedly  attempting  to  investigate  without 
prejudice.  This  plausible  indictment  calls  for  some  defense  on  each 
of  its  specifications. 

Long  before  taking  up  the  investigation  of  osmotic  pressure,  the 
author  had  been  accustomed  to  point  out  certain  defects  of  the  usual 
"volume-normal"  system  of  making  up  solutions,  and  to  maintain  that, 
while  it  was  advantageous  and  correct  for  merely  analytical  purposes, 
it  was  both  disadvantageous  and  illogical  whenever  any  phenomenon 
was  to  be  studied  in  which  the  influence  of  the  solvent  upon  the  solute 
was  involved.  It  was  maintained  that,  in  cases  of  the  latter  kind, 
the  true  concentration  of  a  solution  is  determined  by  the  numerical 
ratio  of  the  molecules  of  the  solute  to  those  of  the  solvent  rather  than 
by  the  number  of  solute  molecules  in  a  given  space.  An  illustration 
frequently  used  for  the  purpose  was  the  case  of  cane  sugar  and  glucose. 
In  a  volume-normal  solution  of  cane  sugar  at  0°,  the  numerical  ratio 
of  solute  to  solvent  molecules  is  about  1  to  44.1,  while  in  a  volume- 
normal  solution  of  glucose  at  the  same  temperature  the  ratio  is  about 
1  to  49.2.  In  other  words,  with  respect  to  the  solvent,  the  cane-sugar 
solution  is  11.5  per  cent  more  concentrated  than  that  of  glucose.  When 
stated  in  terms  of  osmotic  pressure,  the  difference  is  about  3.7  atmos- 
pheres, notwithstanding  the  fact  that  equal  volumes  of  the  two  solutions 
contain  the  same  number  of  solute  molecules. 

Another  illustration  of  the  difficulties  which  are  encountered  when 
volume-normal  solutions  are  employed  was  the  following  example  of  the 
effect  of  what  may  be  called  decimal  dilution.  Suppose  a  0.1  volume- 
normal  solution  of  cane  sugar  to  be  made  up  by  diluting  100  cubic  centi- 
meters of  a  normal  solution  to  1,000  cubic  centimeters.  With  respect 
to  the  relative  numbers  of  solute  molecules  contained  in  equal  volumes, 
the  new  solution  is  one-tenth  as  concentrated  as  that  from  which  it  was 
made,  but  with  respect  to  ratios  of  solute  to  solvent  molecules,  namely, 
1:44.1  and  1:  544.1,  the  concentration  of  the  diluted  solution  is  not 
0.1,  but  0.081  normal. 

When  it  came  to  a  choice  of  systems,  it  was  concluded  by  the  author 
and  his  colleague,  Frazer,  that  the  only  justification  for  the  use  of  the 


WEIGHT-NORMAL   SYSTEM   FOR   SOLUTIONS.  101 

volume-normal  system  was  based  on  the  presumption — already  exten- 
sively abandoned — that  the  phenomenon  of  pressure  in  the  osmotic 
cell  is  due  simply  to  the  bombardment  of  the  membrane  by  the  solute 
molecules.  If  we  had  employed  the  volume-normal  system  for  solutions, 
our  colleagues  could  have  convicted  us,  by  circumstantial  evidence, 
of  being  under  the  dominion  of  a  discarded  conception  of  the  cause 
(and  therefore  of  the  proper  magnitude)  of  osmotic  pressure.  Inas- 
much as  the  obviously  immediate  cause  of  the  pressure  observed  in 
the  cell  is  a  dilution  of  the  solution  within  by  solvent  acquired  from 
without,  the  weight-normal  appeared  to  involve  less  of  hypothesis  and 
to  be  more  rational  than  the  volume-normal  system. 

As  to  the  part  which  is  played  by  the  membrane  in  bringing  about 
this  dilution  of  the  imprisoned  solution,  upon  which  the  existence  of  the 
pressure  in  the  cell  depends,  the  original  idea  of  Graham — that  the 
passage  of  the  aqueous  solvent  through  it  is  due  to  some  sort  of  hydration 
of  the  colloidal  material  of  the  membrane  on  the  side  bathed  by  the 
more  dilute  solution,  and  a  dehydration  on  the  side  covered  by  the 
more  concentrated  solution — has  always  appealed  to  us  as  more  simple 
than  and  quite  as  satisfactory  as  any  of  the  numerous  other  explana- 
tions which  have  been  offered.  It  is  certainly  in  accordance  with  the 
observed  fact  that  the  amount  of  water  which  a  colloid  can  acquire 
and  retain  depends  on  the  concentration  of  the  solution  to  which  it  is 
exposed.  If  Graham's  view  concerning  the  modus  operandi  of  the 
transmission  of  water  is  correct,  the  real  problem  to  be  studied  in  this 
connection  would  seem  to  be  the  dependence  of  the  hydration  of  the 
colloidal  membrane  upon  the  concentration  of  the  solutions  and  upon 
pressure. 

The  course  of  reasoning  which  led  to  the  adoption  of  the  volume  of 
the  solvent  as  the  standard  for  the  computation  of  the  gas  pressure  of 
the  solute  is  quite  elementary  and  appears  to  involve  very  little  of 
hypothesis. 

The  essential  difference  between  a  substance  in  gas  form  and  in 
solution,  which  strikes  one  at  once  and  first  of  all,  is  the  fact  that  the 
molecules  of  a  gas  are  moving  through  space  otherwise  unoccupied, 
while  in  a  solution  the  molecules  of  the  solute  are  moving  through 
space  occupied  by  the  solvent.  The  analogy  of  the  "free  space,"  in  the 
case  of  a  gas,  to  the  free  or  pure  solvent  in  a  solution  is,  in  this  particular, 
obvious  and  unmistakable.  Moreover,  it  was  to  be  presumed  that  the 
space  occupied  by  the  solute  molecules  would  bear,  in  general,  somewhat 
the  same  relation  to  osmotic  pressure  that  the  aggregate  volume  of 
the  gas  molecules  bears  to  the  pressure  of  a  gas — in  other  words,  that 
a  correction  would  have  to  be  employed  for  osmotic  pressure  which  is 
equivalent  to  the  correction  symbolized  by  the  term  b  in  the  equation  of 
van  der  Waals  for  gases.  It  was  also  clear  that,  if  the  pressure  of  a  gas 
could  always  be  computed  on  the  basis  of,  or  referred  to,  the  volume 


102        OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

of  the  free  space,  instead  of  the  total  volume  of  the  gas,  the  correction 
term  b  in  the  equation  of  van  der  Waals  would  be  automatically  elim- 
inated. Such  a  course  is  impracticable  in  the  case  of  gases,  but  easy  in 
that  of  solutions.  The  obviously  equivalent  procedure  in  the  case  of 
solutions  was  to  compute  the  gas  pressure  of  the  solute — with  which 
osmotic  pressure  was  to  be  compared — on  the  basis  of  the  volume  of 
the  solvent,  which  is  known  approximately  wherever  the  weight-normal 
system  of  solutions  is  employed.  The  correction  for  the  volume  of  the 
solute,  which  is  effected  by  adopting  the  weight-normal  system  and  by 
the  practice  of  referring  the  gas  pressure  of  the  solute  to  the  volume  of 
the  solvent,  is,  however,  somewhat  uncertain,  because  the  volume  of 
the  solvent  in  a  solution  is  not  exactly  known.  The  volume  of  a  solu- 
tion is  not  equal  to  the  sum  of  the  volumes  of  the  solvent  and  solute 
in  their  separate  states.  Moreover,  the  volume  of  the  free  solvent  is  lia- 
ble to  diminution  through  combination  of  solvent  with  solute  mole- 
cules, as  in  hydration.  When  a  weight-normal  solution  of  cane  sugar  is 
made  up  at  0°  by  dissolving  a  gram-molecular  weight  of  the  substance 
in  1,000  grams  of  water,  the  volume  of  the  solution  is  less  than  the  sum 
of  the  volumes  of  the  separate  components  by  about  6.7  cubic  centi- 
meters. The  shrinkage  in  the  case  of  glucose  under  identical  conditions 
is  about  6  cubic  centimeters.  It  is  uncertain  how  much  of  this  shrink- 
age is  to  be  ascribed  to  each  of  the  apparently  possible  causes,  i.  e.,  to 
change  in  the  volume  of  the  solvent  itself,  to  change  in  the  state  of 
aggregation  of  the  solute,  and  to  the  formation  of  hydrates.  It  appears 
probable,  however,  that  the  observed  shrinkage  in  volume  is  principally 
due  to  one  or  both  of  the  last  two  causes;  but  only  one  of  these,  namely, 
hydration,  is  known  to  affect  the  volume  of  the  solvent. 

In  regard  to  the  adoption  of  the  volume  of  the  solvent  at  the  tem- 
perature of  maximum  density,  as  the  standard  for  the  computation  of 
the  gas  pressure  of  the  solute,  it  can  only  be  said  that  the  practice  is 
based  on  the  observation  that  the  results  appear  to  be  slightly  more 
harmonious  among  themselves,  when  this  is  done,  than  when  the  gas 
pressure  is  referred  to  the  supposed  volumes  of  the  solvent  at  the  tem- 
perature at  which  the  measurements  of  osmotic  pressure  are  made. 

The  most  amazing  judgment  upon  the  work  of  the  author  and  his 
collaborators  is  that  pronounced  by  Professor  W.  D.  Bancroft,*  who 
says,  in  regard  to  the  practices  elaborated  above: 

"Quite  recently  Morse  and  Frazer  have  shown  that  their  direct  measure- 
ments of  osmotic  pressure  came  out  better  when  the  concentrations  are 
referred  to  a  constant  volume  of  solvent.  They  consider  this  a  discovery  of 
their  own,  quite  overlooking  the  fact  that  they  have  simply  gone  back  to 
van't  Hoff's  original  formulation.  Having  reached  their  conclusion  empiric- 
ally, Morse  and  Frazer  have  also  overlooked  that  their  method  of  expressing 
concentration  contains  the  tacit  assumption  that  there  is  neither  expansion 
nor  contraction  when  the  two  components  are  mixed." 

*Jour.  Phys.  Chem.,  x,  320. 


WEIGHT-NORMAL   SYSTEM   FOR   SOLUTIONS.  103 

In  view  of  the  fact  that  van't  Hoff  had  in  mind  volume-normal 
solutions  only,  and  referred  the  gas  pressure  of  the  solute  always  to 
the  volume  of  the  solution,  the  author  is  unable  to  understand  just  how 
the  adoption  of  the  weight-normal  system  and  the  practice  of  referring 
the  gas  pressure  of  the  solute  to  the  volume  of  the  solvent  constituted  a 
return  to  "varit  Hoff's  original  formulation." 

Furthermore,  it  is  not  clear  what  Professor  Bancroft  has  in  mind 
when  he  says  that  Morse  and  Frazer  have  overlooked  the  fact  that 
their  procedure  "contains  the  tacit  assumption  that  there  is  neither 
expansion  nor  contraction  when  the  two  components  are  mixed."  If  he 
means  that  the  assumption  in  question  is  to  the  effect  that  the  volume 
of  the  solution  is  neither  greater  nor  smaller  than  that  of  the  solvent, 
he  has  apparently  again  failed  to  apprehend  the  clear  distinctions 
between  the  weight-normal  and  the  volume-normal  systems  for  solutions, 
and  has  imputed  to  Morse  and  Frazer  an  equal  confusion  of  ideas. 
If  he  means,  on  the  other  hand,  that  Morse  and  Frazer  have  overlooked 
or  ignored  the  fact  that  the  volume  of  the  solution  is  not  exactly  equal 
to  the  sum  of  the  volumes  of  solvent  and  solute  separately,  he  is  quite 
misinformed  as  to  the  state  of  their  knowledge  of  solutions,  and  as  to 
their  attitude  of  mind  toward  the  volume  relations  in  question. 

It  was  realized  in  the  beginning  that  the  volume  relations  of  solvent, 
solute,  and  solution  constitute  an  important  phase  of  the  subject  under 
investigation,  and  that  they  should  be  determined  with  the  utmost 
practicable  precision.  Accordingly,  almost  simultaneously  with  the 
measurement  of  the  osmotic  pressure  of  cane-sugar  and  glucose,  there  was 
begun  a  very  careful  parallel  investigation  of  the  volumes  of  the  various 
weight-normal  solutions  of  those  substances.  The  work  at  0°  was  fin- 
ished, but  that  at  the  higher  temperatures  is  still  incomplete. 

An  example  of  the  kind  of  information  which  was  sought  is  given 
in  Tables  6  and  7. 

The  important  question — considered  in  its  bearing  upon  the  practice 
of  referring  the  gas  volume  of  the  solute  to  the  volume  of  the  solvent — 
is  not  what  is  the  total  contraction  which  is  observed  when  the  com- 
ponents of  a  solution  are  brought  together,  but  to  what  extent  does  the 
contraction  modify  the  volume  of  the  solvent  itself?  Any  shrinkage  or 
expansion  which  may  be  due  to  the  fact  that  the  solute  monopolizes 
less  or  more  space  in  the  solution  than  in  its  previous  separate  state 
does  not  necessarily  involve  the  volume  of  the  solvent.  Contraction 
due  to  the  formation  of  hydrates  in  solution  does  involve  the  solvent, 
and  it  undoubtedly  diminishes  its  volume  and  concentrates  the  solution 
in  what  may  be  called  the  weight-normal  sense.  This  concentration 
of  the  solution  through  hydration  of  the  solute  must  express  itself  in 
the  form  of  an  equivalent  increase  in  osmotic  pressure.  An  "ideal" 
solution,  in  the  "weight-normal  sense"  is  one  in  which  the  solvent  has 
the  same  volume  as  in  the  separate  state  and  is  otherwise  uninvolved. 


104 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


If  it  can  once  be  determined  what  rule  or  law  governs  the  magnitude 
of  the  osmotic  pressure  of  the  solute  in  such  "ideal"  solutions,  it  will 
be  practicable  to  study  effectively  the  subject  of  hydration  in  aqueous 
solutions.  No  other  method  of  equal  comprehensiveness  and  promise 
is  available,  except  perhaps  the  vapor  pressure  method,  and  that,  in  its 
present  imperfect  condition,  is  not  adapted  to  the  investigation  of  hydra- 
tion. It  is  to  be  hoped  that  the  study  of  solutions  at  high  temperatures — 
at  temperatures  so  high  as  to  preclude  the  existence  of  hydrates — will 

TABLE  6. — Volume  of  weight-normal  solutions  of  glucose  at  0°. 
[Sp.  gr.  glucose  at  0°  =  1.5567.] 


Concentration. 

Volume  of 
solvent. 

Volume  of 
solute. 

Sum. 

Volume  of 
solution. 

Difference. 

Contraction. 

c.c. 

c.c. 

c.c. 

c.c. 

c.c. 

p.ct. 

0.1 

1000.13 

11.48 

1011.61 

1010.93 

0.68 

0.07 

0.2 

" 

22.96 

1023.09 

1021.70 

1.39 

0.14 

0.3 

" 

34.45 

1034.58 

1032.55 

2.03 

0.20 

0.4 

" 

45.93 

1046.06 

1043.42 

2.64 

0.25 

0.5 

" 

57.41 

1057.54 

1054.29 

3.25 

0.31 

0.6 

" 

68.89 

1069.02 

1065.22 

3.80 

0.36 

0.7 

" 

80.37 

1080.50 

1076.14 

4.36 

0.40 

0.8 

" 

91.81 

1091.99 

1087.06 

4.93 

0.45 

0.9 

" 

103.34 

1103.47 

1097.94 

5.53 

0.50 

1.0 

114.82 

1114.95 

1108.92 

6.03 

0.54 

TABLE  7. — Volume  of  weight-normal  solutions  of  cane  sugar  at  0°. 
[Sp.  gr.  cane-sugar  at  0°  =1.59231.] 


Concentration. 

Volume  of 
solvent. 

Volume  of 
solute. 

Sum. 

Volume  of 
solution. 

Difference. 

Contraction. 

c.c. 

c.c. 

c.c. 

c.c. 

c.c. 

p.  ct. 

0.1 

1000.13 

21.328 

1021.458 

1020.73 

0.728 

0.07 

0.2 

" 

42  .  656 

1042  .  786 

1041.25 

1.536 

0.15 

0.3 

" 

63.984 

1064.114 

1061.80 

2.314 

0.22 

0.4 

" 

85.312 

1085.442 

1082.38 

3.062 

0.28 

0.5 

" 

106.640 

1106.770 

1103.01 

3.760 

0.34 

0.6 

" 

127.968 

1128.098 

1123.70 

4.398 

0.39 

0.7 

" 

149  .  296 

1149.426 

1144.39 

5.036 

0.44 

0.8 

" 

170.624 

1170.754 

1165.13 

5.624 

0.48 

0.9 

" 

191.952 

1192.082 

1185.91 

6.172 

0.52 

1.0 

213.280 

1213.410 

1206.69 

6.720 

0.55 

eventually  reveal  the  simplest  forms  of  the  laws  governing  the  osmotic 
pressure  in  aqueous  solutions.  If  these  are  once  established,  we  can 
then  measure  hydration  at  lower  temperatures  by  the  apparent  abnor- 
malities of  the  osmotic  pressure.  It  is  not  to  be  ignored,  of  course,  that 
other  factors  than  hydration  may  assert  themselves  at  the  lower  temp- 
eratures and  obscure,  to  some  extent,  the  results. 

The  reasons  given  or  implied  in  the  foregoing  statement  are  those 
which  decided  the  author  and  his  collaborators  to  adopt,  in  the  be- 
ginning, the  weight-normal  system  of  solutions  for  the  measurement  of 


WEIGHT-NORMAL   SYSTEM   FOR   SOLUTIONS.  105 

osmotic  pressure.  They  have  never  made  any  claim  to  the  discovery 
of  this  system,  as  has  been  intimated  by  one  critic  of  their  work.  Such 
a  claim  would  have  been  absurd  in  the  light  of  the  fact  that  the  "weight- 
normal"  system  was  the  obvious  and  the  already  known  alternative  of 
the  "volume-normal,"  or  more  usual,  system  of  making  up  solutions. 

The  question  now  to  be  answered  is  whether  the  experimental  data, 
as  far  as  they  have  been  acquired  up  to  the  present  time,  do,  or  do  not, 
appear  to  justify  the  wisdom  of  the  choice  which  was  made  and  to 
call  for  its  continuance  in  use.  The  strongest  evidence  which  could 
be  adduced  in  favor  of  any  system  would  be  the  fact  that,  under  it, 
the  osmotic  pressures  of  the  solutions  appear  to  conform  to  a  definite 
temperature  coefficient  and  to  bear  some  definite  relation  to  concen- 
tration. It  is  not  at  all  necessary,  in  order  to  give  weight  to  the 
evidence,  that  the  relations  in  question  shall  be  found  to  conform  to 
the  laws  of  Gay-Lussac  and  of  Boyle  for  gases.  Evidence  pointing 
equally  clearly  to  the  existence  of  other  laws  than  these  would  be 
quite  as  convincing.  The  facts  are,  however,  that,  under  the  weight- 
normal  system,  all  the  reliable  osmotic  evidence  thus  far  gathered 
points  emphatically  either  to  a  substantial  conformity  with  the  laws 
of  Gay-Lussac  and  of  Boyle  for  gases,  or  to  a  species  of  non-conformity 
which  is  rationally  and  adequately  explainable  on  the  supposition  that, 
at  moderate  temperatures,  some  of  the  solutes  are  hydrated.  A  brief 
resume  is  given  below  of  the  established  facts  which  bear  upon  the 
question  of  the  obedience  of  osmotic  pressure  in  aqueous  solutions  to 
the  laws  of  Gay-Lussac  and  of  Boyle. 

(1)  It  has  been  shown  that  in  all  solutions  of  cane  sugar,  from  0.1 
to  1.0  weight-normal,  the  ratio  of  the  observed  osmotic  to  the  estimated 
gas  pressure  of  the  solute  is  constant  for  each  concentration  between 
0°  and  25°.     This  proves  that,  between  the  specified  limits  of  concentra- 
tion and  temperature,  the  osmotic  pressure  of  cane-sugar  solutions 
obeys  the  law  of  Gay-Lussac  for  gases. 

(2)  The  ratio  in  question  exceeds  unity  in  every  instance.     This 
suggests,  of  course,  a  concentration  of  the  solutions  through  a  with- 
drawal of  some  of  the  solvent  for  the  purpose  of  hydrating  the  solute. 
If  hydration  exists,  it  must  be  constant  in  quantity  for  each  concen- 
tration of  solution  within  the  given  limits  of  temperature;  for,  other- 
wise, the  law  of  Gay-Lussac  could  not  hold. 

(3)  The  osmotic  pressure  of  cane-sugar  solutions,  between  0°  and  25°, 
are  not  proportional  to  the  quantities  of  the  solute.     In  other  words,  the 
ratio  of  osmotic  to  gas  pressure  varies  from  concentration  to  concentra- 
tion, though,  as  stated  under  (1),  it  is  strictly  constant  for  any  given 
concentration.     This  leaves  the  applicability  of  Boyle's  law  in  doubt, 
but  does  not  demonstrate  its  inapplicability;  for  the  phenomenon  may 
be  due  to  differences  among  the  various  concentrations  of  solution  in 
respect  to  the  degree  of  hydration  which  they  have  severally  suffered. 


106       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

(4)  When  solutions  of  cane  sugar  are  heated  to  a  temperature  above 
25°,  the  ratios  of  osmotic  to  gas  pressure — which  are  all  above  unity 
and  are  constant  for  each  concentration  at  lower  temperatures — begin 
to  decline.     The  decrease  in  the  ratio  with  rising  temperature  is  rela- 
tively more  rapid  in  dilute  solutions  than  in  concentrated  ones.     Such 
conduct  on  the  part  of  solutions  is  indicative  of  the  presence  in  them 
of  dissociating  hydrates. 

(5)  The  decline  in  the  ratio  of  osmotic  to  gas  pressure,  which  begins 
a  little  above  25°,  continues  until,  at  some  temperature  which  is  char- 
acteristic for  each  concentration,  it  becomes  unity.     This  shows  that, 
at  these  temperatures,  the  osmotic  pressures  of  all  the  solutions  conform 
both  to  the  law  of  Gay-Lussac  and  to  that  of  Boyle. 

(6)  The  work  upon  solutions  of  cane  sugar,  between  the  boiling- 
point  of  the  solvent  and  the  temperatures  at  which  the  ratio  of  osmotic 
to  gas  pressure  becomes  unity  for  the  several  concentrations,  has  not 
been  finished,  but  there  is  already  in  hand  considerable  evidence  to  the 
effect  that  a  ratio,  having  once  become  unity  at  some  temperature,  does 
not  further  decline  at  still  higher  temperatures. 

(7)  The  conduct  of  glucose  solutions  differs  somewhat  but  not  wholly 
from  that  of  cane-sugar  solutions:  (a)  At  0°  the  ratio  of  osmotic  to 
gas  pressure  is  greater  than  unity,  which  again  suggests  hydration.    The 
ratio  is,  however,  the  same  for  all  concentrations  of  solution.     In  other 
words,  the  osmotic  pressures  are  proportional  to  concentration.     This 
means  that  they  conform  to  the  law  of  Boyle.    (6)  At  some  temperature 
above  0°,  but  below  10°,  the  ratio  begins  to  decline,  which  suggests  the 
presence  of  dissociating  hydrates.     At  10°,  half  the  difference  between 
the  observed  ratio  at  0°  and  unity  has  already  disappeared.     But  the 
ratio  is  still  the  same  for  all  concentrations,  showing  that  the  law  of 
Boyle  holds  at  10°,  as  well  as  at  0°.     (c)  At  25°  and  also  at  30°,  40°,  and 
50°  the  ratio  of  osmotic  to  gas  pressure  is  unity  for  all  concentrations 
from  0.1  to  1.0  weight-normal,  proving  that  at  these  temperatures  the 
osmotic  pressure  of  glucose  solutions  obeys  both  of  the  gas  laws. 

(8)  The  ratio  of  osmotic  to  gas  pressure  in  all  solutions  of  mannite 
is  unity  at  10°,  20°,  30°,  and  40°.     Its  value  at  other  temperatures  has 
not  been  ascertained. 

The  mistaken  impression  that  the  author  and  his  collaborators 
are  engaged  in  an  endeavor  to  demonstrate  that  the  gas  laws  apply 
generally  to  osmotic  pressure  is  probably  due  to  the  emphasis  which 
has  frequently  been  laid  upon  the  relations  pointed  out  above.  The 
truth  is,  however,  that  they  have  limited  their  discussions  to  the  few 
facts  established  by  themselves,  and  have  only  sought  to  formulate 
the  more  obvious  relations  of  their  own  experimental  data.  If  any  rule 
proposed  by  them  as  apparently  fitting  their  experimentally  acquired 
facts  has  been  found  susceptible  of  a  concise  mathematical  expression, 
it  has  not  thereby  acquired,  in  their  estimation,  any  additional  merit 


WEIGHT-NORMAL   SYSTEM   FOR   SOLUTIONS. 


107 


or  utility,  or,  least  of  all,  the  character  of  a  general  equation.  It  has 
still  remained — despite  its  more  impressive  appearance  in  mathematical 
dress — simply  a  rule,  the  question  of  whose  validity  was  to  be  strictly 
limited  to  the  already  known  and  fully  accredited  facts,  and  which, 
therefore,  was  subject  to  modification  as  the  number  of  established  facts 
increased.  The  direct  measurement  of  osmotic  pressure  is  a  task  of 
supreme  difficulty,  and  those  who  would  undertake  it  effectively  should 
qualify  themselves  for  the  enterprise  by  discarding  all  convictions  as 
to  whither  their  labors  may  lead  them. 

Probably  it  will  be  generally  conceded  that  the  weight-normal  is  the 
simpler  and  more  rational  system  for  the  statements  of  the  freezing- 
point  depressions  of  aqueous  solutions.  We  have  determined  these  in 
all  of  the  concentrations  of  solution  of  cane  sugar  and  glucose  which 
have  been  employed  for  the  measurement  of  osmotic  pressure,  and 
they  are  given  in  Table  8,  together  with  the  corresponding  molecular 
depressions  of  the  freezing-points. 

TABLE  8. — Cane  sugar  and  glucose. — Depression  of  the  freezing-points 
of  weight-normal  solutions. 


Cane-sugar. 

Glucose. 

Concentration. 

Depression. 

Molecular 
depression. 

Depression. 

Molecular 
depression. 

degrees. 

degrees. 

degrees. 

degrees. 

0.1 

0.195 

1.95 

0.192 

1.92 

0.2 

0.393 

1.96 

0.386 

1.92 

0.3 

0.584 

1.95 

0.576 

1.92 

0.4 

0.784 

1.96 

0.762 

1.91 

0.5 

0.983 

1.97 

0.952 

1.91 

0.6 

1.190                1.98 

1.147 

1.91 

0.7 

1.390 

1.99 

1.337 

1.91 

0.8 

1.621 

2.02 

1.528 

1.91 

0.9 

1.829 

2.03 

1.720 

1.91 

1.0 

2.066 

2.07 

1.918 

1.92 

Table  9  is  added  in  order  to  illustrate  and  emphasize  the  limitations 
of  the  freezing-point  method  as  a  means  for  the  determination  of 
osmotic  pressure  in  aqueous  solutions. 

The  direct  measurement  of  osmotic  pressure  is  often  deprecated  on 
account  of  the  great  difficulties  which  are  encountered;  and  it  is  fre- 
quently asserted,  with  an  air  of  thorough  conviction,  that  there  are  other 
methods  available  which  are  more  accurate  and  much  easier.  If  this 
were  really  true,  we  should  have  emerged  long  ago  from  our  present 
lamentable  state  of  ignorance  with  regard  to  the  osmotic  pressure  of 
solutions.  The  very  fact  that  we  can  not  yet  make  a  safe  prediction  as 
to  the  magnitude  of  osmotic  pressure  in  any  aqueous  solution  which  has 
not  been  extensively  investigated  by  the  direct  method  is  proof  enough 
that  these  other  "  more  accurate  and  easier  methods"  have  failed  to  render 
the  service  of  which  they  are  said  to  be  capable. 


108 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


The  methods  which  are  always  cited  in  this  connection  are  three 
in  number — namely,  the  freezing-point,  the  boiling-point,  and  the  vapor- 
tension  methods.  It  is  known  that  the  depression  of  the  freezing-point 
of  an  aqueous  solution  will  give  us  approximately  its  osmotic  pressure 
within  a  very  limited  region  of  temperature  which  includes  the  freezing- 
point  itself.  So  much  has  been  experimentally  proved.  It  is  probably 
true  also — though  it  has  not  yet  been  demonstrated  by  direct  measure- 
ments— that  the  osmotic  pressure  of  an  aqueous  solution  in  the  imme- 
diate vicinity  of  its  boiling-point  can  be  derived  from  the  elevation  of 
the  boiling-point.  But  how  about  the  osmotic  pressures  of  the  solution 
throughout  that  relatively  much  larger  temperature  area,  between  the 
freezing  and  boiling  points,  within  which  a  variable  hydration  may  exist, 
or  other  molecular  influences  than  those  concerned  in  the  formation 
of  hydrates  may  come  into  play  and  modify  osmotic  pressure? 

TABLE  9. — Cane  sugar.   Comparison  of  osmotic  pressures  calculated  from  freezing-points 
with  those  directly  determined. 


0 

5 

1( 

)° 

11 

5° 

2 

3° 

• 

25° 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

0.1 
0.2 
0.3 
0.4 
0.5 
0.6 
0.7 
0.8 
0.9 
1.0 

2.35 
4.72 
7.04 
9.44 
11.86 
14.30 
16.77 
19.45 
21.99 
24.91 

2.46 
4.72 
7.09 
9.44 
11.90 
14.38 
16.89 
19.48 
22.13 
24.83 

2.39 
4.81 
7.17 
9.61 
12.08 
14.56 
17.07 
19.81 
22.39 
25.37 

2.45 
4.82 
7.20 
9.61 
12.10 
14.61 
17.21 
19.82 
22.48 
25.28 

2.43 
4.89 
7.30 
9.78 
12.29 
14.82 
17.37 
20.16 
22.80 
25.82 

2.50 
4.89 
7.33 
9.79 
12.30 
14.86 
17.50 
20.16 
22.88 
25.69 

2.48 
4.98 
7.43 
9.95 
12.51 
15.08 
17.69 
20.52 
23.20 
26.28 

2.54 
4.99 
7.48 
9.95 
12.55 
15.14 
17.82 
20.54 
23.31 
26.19 

2.52 
5.06 
7.56 
10.13 
12.73 
15.35 
17.99 
20.88 
23.60 
26.74 

2.59 
5.06 
7.61 
10.14 
12.75 
15.39 
18.13 
20.91 
23.72 
26.64 

2.56 
5.15 
7.69 
10.30 
12.94 
15.61 
18.30 
21.23 
24.01 
27.20 

2.63 
5.15 
7.73 
10.30 
12.94 
15.63 
18.44 
21.25 
24.13 
27.05 

3 

)° 

41 

)° 

5( 

)° 

6( 

)° 

7 

3° 

! 

}0° 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

Calc. 

Det. 

0.1 

2.61 

2.47 

2.69 

2.56 

2.78 

2.64 

2.86 

2.72 

0.2 

5.24 

5.04 

5.41 

5.16 

5.58 

5.28 

5.76 

5.44 

0.3 

7.82 

7.65 

8.07 

7.84 

8.33 

7.97 

8.59 

8.14 

0.4 

10.47 

10.30 

10.82 

10.60 

11.17 

10.72 

11.51 

10.87 

0.5 

13.16 

12.98 

13.60 

13.36 

14.03 

13.50 

14.47 

13.66 

14  90 

13.99 

0.6 

15.87 

15.71 

16.40 

16.15 

16.92 

16.32 

17.45 

16.54 

17.91 

16.82 

0  7 

18.61 

18.50 

19.23 

18.93 

19.84 

19.20 

20.46 

19.40 

21.07 

19.57 

0.8 
0.9 
1.0 

21.59 
24.41 
27.66 

21.38 
24.23 
27.22 

22.30 
25.22 

28.57 

21.80 
24.74 
27.70 

23.02 
26.02 
29.48 

22.12 
25.12 
28.21 

23.73 
26.83 
30.40 

22.33 
25.27 

28.37 

24.44 
27.64 
-31.31 

22.57 
25.56 
28.62 

25.16 
28.44 
32.23 

23.06 
25.92 

28.82 

The  osmotic  pressures  given  for  cane  sugar  in  Table  9  have  been 
calculated  from  the  depressions  of  the  freezing-points  of  the  solutions 
(Table  8),  and  they  are  there  compared  with  the  pressures  which  were 
obtained  by  direct  measurement.  As  seen  in  Table  9,  the  agreement  is 
satisfactory  at  all  temperatures  not  exceeding  25° ;  but  the  two  sets  of 
values  are  thoroughly  and  increasingly  discordant  at  all  higher  tempera- 
tures. It  appears,  therefore,  that  the  osmotic  pressure  of  cane-sugar 
solutions  can  be  calculated  up  to  25°  from  the  depressions  of  the  freezing- 


WEIGHT-NORMAL   SYSTEM   FOR   SOLUTIONS.  109 

points,  but  not  at  higher  temperatures.  In  other  words,  they  can  be  calcu- 
lated correctly  up  to  the  temperature  above  which  the  previously  constant 
ratios  of  osmotic  to  gas  pressure  were  found  to  begin  to  decline  in  value, 
and  no  further.  So  far  as  known  at  the  present  time,  all  the  osmotic 
pressures  of  cane-sugar  solutions  at  and  above  the  temperatures  at  which 
the  various  ratios  of  osmotic  to  gas  pressure  become  unity,  can  be  correctly 
calculated  from  a  normal  molecular  depression  of  the  freezing-points,  i.e., 
from  a  supposed  molecular  depression  of  about  1.85°.  The  tempera- 
ture area  within  which  the  depressions  of  the  freezing-points  can  be 
employed  for  the  determination  of  osmotic  pressure  is  much  smaller 
in  the  case  of  glucose  than  in  that  of  cane  sugar.  The  boiling-point 
method  of  determining  osmotic  pressure  is  also  hampered  by  restric- 
tions. It  is  limited  in  its  applicability  to  a  wholly  unknown  tem- 
perature area — "unknown"  because  no  one  can  predict  at  what  lower 
temperature  hydration,  or  some  equivalent  phenomenon,  will  manifest 
itself.  If  the  freezing  and  boiling  points  of  a  solution  were  both  nor- 
mal, it  would  probably  be  practicable  to  calculate  from  either  of  them 
the  osmotic  pressure  at  any  intermediate  temperature.  But  if  one  of 
them  is  abnormal — and  one  or  the  other  is  usually  abnormal  in  the 
case  of  aqueous  solutions — this  can  not  be  done. 

But  the  vapor-tension  method  of  determining  osmotic  pressure — 
unlike  the  freezing  and  boiling  point  methods — is  not  of  restricted  appli- 
cability. It  can  be  employed  at  all  temperatures  between  the  freezing 
and  boiling  points  of  solutions.  All  that  has  been  said  in  praise  of  the 
comprehensive  character  of  the  vapor-tension  method  is  true  enough — 
theoretically;  but  if  anyone  who  is  enthusiastic  for  it  will  once  attempt 
to  apply  it,  he  will  soon  discover  for  himself  some  of  the  reasons  why  it 
has  not  come  into  general  use  for  the  measurement  of  osmotic  pressure. 
The  most  needed  agent  for  the  investigation  of  solutions  at  the  present 
time  is  a  really  practicable  precision-method  for  the  measurement  of 
vapor  pressure  at  all  temperatures. 

In  the  foregoing  pages,  the  author  has  frequently  spoken  of  "hydra- 
tion" as  something  which  may  account  for  apparently  abnormal  con- 
duct on  the  part  of  solutions.  He  has  employed  this  explanation  of 
excessive-constant  and  excessive-declining  ratios  of  osmotic  to  gas  pressure 
because  it  is  the  simplest  and  most  statisfactory  one  at  hand,  and  not 
because  he  is  fully  convinced  of  its  entire  correctness. 


CHAPTER  VI. 
CANE  SUGAR. 

PRELIMINARY  DETERMINATIONS  OF  OSMOTIC  PRESSURE. 

The  numerous  determinations  of  the  osmotic  pressure  of  cane  sugar 
which  are  to  be  presented  in  this  report  will  be  classified  as  preliminary 
or  final,  according  as  they  were  made  before  or  after  the  method  of 
measuring  the  force  had  been  developed  to  a  point  where,  in  the  author's 
judgment,  full  credence  was  to  be  given  to  the  results. 

The  final  arrangements  for  the  measurement  of  osmotic  pressure 
which  have  been  described  in  the  earlier  chapters,  and  the  methods 
of  manipulation  which  will  be  discussed  to  some  extent  hereafter,  were, 
as  a  rule,  the  products  of  a  slow  growth.  In  a  general  way,  the  whole 
history  of  the  investigation  may  be  divided  into  three  periods,  as  follows : 
First,  a  period  of  four  years,  in  which  the  attention  of  the  writer  and 
his  co-workers  was  given  almost  exclusively  to  the  task  of  perfecting 
the  porous  wall  of  the  cell;  second,  a  period  of  nearly  equal  duration, 
in  which  they  were  measuring  osmotic  pressure  with  a  view  to  discover- 
ing and  eliminating  the  sources  of  error  in  the  method;  third,  the  period 
within  which — owing  to  the  absence  of  any  large  sources  of  error — the 
results  are  regarded  as  reliable  in  a  high  degree. 

During  the  second  or  evolutionary  period,  eight  series  of  quantitative 
measurements  of  the  osmotic  pressure  of  cane  sugar  were  made,  and 
three  series  of  measurements  on  solutions  of  glucose.  The  present 
chapter  gives  an  account  of  the  work  upon  cane  sugar. 

The  value  of  the  results  increases  quite  continuously  from  the  first 
to  the  last  series  in  the  proportion  in  which  it  was  found  practicable 
to  diminish  or  suppress  sources  of  error.  Two  sources  of  error — ther- 
mometer effects,  and  dilution  of  the  cell  contents  during  or  after  a  meas- 
urement of  pressure — were  found  to  exceed  all  others  in  importance  and 
to  vitiate  the  results  more  than  all  other  defects  of  the  method.  They 
were  also  the  most  difficult  to  deal  with.  In  fact,  the  whole  four  years 
may  be  said  to  have  been  devoted  to  their  elimination. 

The  "thermometer  effects"  have  been  sufficiently  discussed  in  a 
former  chapter.  They  were  due,  of  course,  to  the  imperfections  of  the 
earlier  arrangements  for  the  maintenance  of  temperature. 

The  dilution  of  the  cell  contents  (which,  at  first  sight,  would  naturally 
be  ascribed  to  leakage  of  the  membranes)  was  found  to  be  due  to  two 
causes:  First,  to  an  acquisition  of  solvent  during  the  closing  and  the 
opening  of  the  cells;  and  second,  to  an  enlargement  of  cell  capacity 
under  pressure.  Accordingly,  during  the  whole  of  the  period  within 

ill 


112      OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

which  the  so-called  preliminary  measurements  fall,  the  chief  concern 
of  the  writer  and  of  his  collaborators  was  to  perfect  the  means  of 
maintaining  constant  temperature,  to  lessen  the  time  required  for  the 
opening  and  closing  of  the  cells,  and  to  develop  a  cell  whose  capacity 
could  not  increase  under  pressure.  Minor  sources  of  error,  of  which 
there  are  many,  were  not  neglected,  but  the  attention  given  them  was 
strictly  proportional  to  the  relative  magnitude  of  their  effects  upon 
the  precision  of  the  results.  After  thermometer  effects  and  dilution, 
more  attention  was  given  to  the  improvement  of  the  manometers  than 
to  any  other  feature  of  the  method. 

The  second  period  begins  with  Series  I,  in  which  the  fluctuations 
in  bath  temperature  amounted,  in  some  instances,  to  whole  degrees, 
and  in  which  the  dilution  of  the  cell  contents,  though  unknown,  must 
have  been  very  large;  and  it  closes  with  Series  VIII,  which  was  carried 
through  without  any  material  variation  in  bath  temperature  and  with- 
out any  dilution  of  the  cell  contents  which  could  be  detected  by  the 
polariscope. 

The  part  played  by  the  first  eight  series  in  the  evolution  of  the  method 
gives  them  great  importance  in  the  history  of  the  investigation,  but 
the  actual  results  of  the  measurements  are  to  be  considered  and 
appraised  merely  as  tests  of  progress  in  the  development  of  the  method. 
They  will  be  treated  as  such  throughout  by  the  writer,  and  not  dis- 
cussed, to  any  considerable  extent,  with  reference  to  the  light  which 
they  throw  upon  the  true  osmotic  pressure  of  cane-sugar  solutions. 

SERIES  I.* 

The  cell  employed  in  this  series  was  that  seen  in  Figure  7,  page  19. 
The  closing  and  the  opening  of  such  a  cell  are  difficult  and  strenuous 
performances,  which  require  the  cooperation  of  two  experienced  persons. 
Both  operations  will  be  briefly  described  because  of  the  bearing  they 
have  upon  the  dilution  of  the  cell  contents  which  it  was  so  difficult  to 
suppress. 

In  closing,  one  of  the  operators  (No.  1)  holds  in  one  hand  the  filled 
cell,  which  is  covered  with  a  piece  of  very  thin  rubber  tubing  to  prevent 
any  soiling  of  the  outside  of  the  cell  by  the  overflow  of  the  solution  or 
by  the  hand.  With  the  other  hand  he  holds  and  manipulates  the 
manometer,  the  nut  (h,  Figure  7)  resting  upon  the  back  of  the  hand 
which  grips  the  manometer  by  the  rubber  stopper  (k).  The  duty  of 
operator  No.  2  is  to  manipulate  the  "fang"  (Figure  8),  by  means  of 
which  the  rubber  is  worked  into  the  cell  and  an  equal  amount  of  the 
solution  is  let  out  of  it;  and  afterwards  to  wrap  and  tie  with  twisted 
and  waxed  shoemakers'  thread  the  exposed  part  of  the  stopper  when 
it  has  been  forced  to  a  sufficient  depth  into  the  glass  tube 


*Measurements  by  H.  N.  Morse  and  J.  C.  W.  Frazer.     Am.  Chem,  Jour.,  xxxiv,  1. 


CANE   SUGAR.  113 

No.  1  places  the  stopper  in  position  for  entrance  over  the  mouth  of 
the  glass  tube  and  pushes  forward  with  all  his  strength.  Since  the 
tube  at  the  mouth  is  considerably  constricted,  the  stopper  does  not 
enter.  No.  2  now  introduces  the  fang  and  works  the  rubber  little  by 
little  through  the  narrow  opening  until  enough  of  it  has  been  buried 
in  the  glass  tube.  In  the  meantime,  No.  1  turns  and  twists  the  stopper 
in  any  direction  which  seems  likely  to  promote  the  progress  of  No.  2. 

From  the  time  when  the  rubber  stopper  first  enters  the  glass  tube 
until  the  nut  (h)  is  brought  down  upon  it  and  secured  by  the  brass 
collar  (g),  there  must  be  no  relaxation  of  the  pressure  exerted  by  No.  1. 
Otherwise  air  will  enter  the  cell  through  the  groove  on  the  under  side 
of  the  fang;  or  if,  as  in  the  earlier  work,  no  safety  reservoir  has  been 
provided  for  the  gas  in  the  manometer,  some  of  it  may  escape  from 
the  calibrated  portion  of  the  instrument.  When,  therefore,  the  stopper 
has  been  introduced  and  the  fang  withdrawn,  and  it  is  necessary  for 
No.  1  to  remove  his  fingers  from  the  sides  to  the  top  of  the  stopper  in 
order  to  make  room  for  the  winding  operations  of  No.  2,  the  stopper 
is  seized  and  held  by  the  latter  until  the  former  has  his  fingers  firmly 
fixed  in  their  new  position.  Similar  aid  is  required  from  No.  2  when, 
with  the  winding  of  the  exposed  part  of  the  stopper  completed,  it  is 
necessary  for  No.  1  to  remove  his  fingers  from  the  top  of  the  stopper  to 
the  top  of  the  nut  (h).  After  this  change  of  position  has  been  effected, 
any  desired  initial  pressure  is  brought  upon  the  contents  of  the  cell  by 
turning  up  the  brass  collar  (0)  on  the  nut  (h). 

When  a  cell  is  to  be  opened,  No.  1  holds  the  apparatus  as  in  closing 
and  attempts  to  withdraw  the  stopper  by  pulling,  while  No.  2  admits 
air  to  the  contents  of  the  cell  by  inserting  the  fang  between  the  rubber 
and  the  glass  tube.  But  the  simple  admission  of  air  by  No.  2  and  the 
simultaneous  efforts  of  No.  1  do  not  suffice  for  the  removal  of  the 
stopper.  It  is  necessary  for  No.  2  to  work  the  rubber,  little  by  little, 
out  of  the  glass  tube  with  the  fang,  while  No.  1  maintains  a  steady  pull 
upon  the  stopper. 

It  will  be  seen  that  both  the  closing  and  the  opening  of  the  cells 
required  considerable  time.  In  the  beginning,  each  operation  con- 
sumed about  15  minutes. 

The  first  arrangements  for  the  maintenance  of  temperature  were 
crude  and  wholly  inadequate.  The  bath  employed  in  the  measure- 
ments of  Series  I  is  shown  in  Figure  46.  It  consisted  of  a  double- 
walled  box  with  two  front  doors  (a  and  c).  The  former  (a)  had  a 
narrow  plate-glass  window  (6),  through  which  the  height  of  the  mercury 
in  the  manometers  and  thermometers  could  be  read  without  opening 
the  inner  door.  The  outer  door  (c)  had  in  its  central  part  a  smaller 
door  (d),  which  was  of  the  same  size  as  the  window  (6).  The  spaces 
between  the  outer  and  inner  walls  of  the  box  were  filled  with  hair. 
Between  the  doors  (a  and  c)  a  hair  pad  was  placed,  which  exactly 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


filled  the  space  except  over  the  window  (6).  The  window  was  covered 
by  a  separate  pad,  which  could  be  introduced  or  withdrawn  through 
the  small  outer  door  (d).  The  top  of  the  box  was  removable. 

The  cell — with  a  long  thermometer  whose  bulb  was  immersed  in 
the  water  surrounding  the  cell — was  placed  in  the  box  and  packed 
with  hair,  except  where  it  was  necessary  to  leave  vacant  spaces  for  the 
purpose  of  reading  the  instruments.  The  exterior  of  the  box  was 
protected,  during  an  experiment,  by  coverings  of  thick  hair-felt,  by 
woolen  cloths,  and  even  by  sheepskins.  Care  was  also  taken  to  mod- 


FIG.  46. — First  bath  employed  for  measurement  of  osmotic  pressure. 
Double- walled,  and  filled  between  with  hair. 

(a)  Inner  door;  (6)  glass  window;  (c)  outer  door;  (d)  door  of  size  of  (b). 

erate  somewhat  the  extremes  of  temperature  in  the  room  in  which 
the  bath  was  located.  No  attempt  was  made  to  maintain  some 
precise  temperature,  e.  g.,  20°.  The  cell  was  filled  and  placed  in  the 
bath,  and  was  protected  in  the  manner  described,  at  the  temperature 
of  the  room.  In  a  general  way,  the  origin  and  the  modus  operandi 
of  thermometer  effects  were  understood,  but  it  was  foreseen  that  their 
existence  depends  on  the  rate  at  which  the  solvent  can  pass  in  either 
direction  through  the  membrane  to  compensate  the  changes  in  the 
volume  of  the  inclosed  solution  which  are  due  to  fluctuations  of  tem- 
perature. It  was  believed,  moreover,  that,  in  solutions  protected  as 


CANE    SUGAR. 


115 


were  ours,  the  changes  in  temperature  would  be  so  moderate  and 
gradual  that  thermometer  effects  were  not  to  be  apprehended;  in  other 
words,  that  the  solutions  would  at  all  times  exhibit  their  true  osmotic 
pressure,  whatever  their  temperatures  might  be.  It  was  soon  discov- 
ered, however,  that  the  speed  with  which  the  membrane  is  accustomed 
to  compensate  changes  in  volume  through  dilution  or  concentration 
of  the  solution  had  been  greatly  overestimated.  There  is  no  doubt, 
therefore,  that  the  thermometer  effects  in  Series  I  were  very  large. 

The  material  employed  in  Series  I  to  VIII,  inclusive,  was  the  purest 
obtainable  "rock  candy."  It  was  not  recrystallized,  but  was  analyzed 
and  examined  by  the  polariscope,  and  was  judged  to  be  sufficiently  pure 
for  preliminary  experiments. 

TABLE  10.— Cane  sugar,  Series  I. 


Concentration. 

Temperature. 

Observed 
osmotic 
pressure. 

Mean 
osmotic 
pressure. 

Gas 
pressure. 

Calculated 
molecular 
weight. 

0.05 

degrees. 
20.36  to  21.  24 
20.20       20.90 

1.28 
1.25 

}       1.27 

1.21 

327.50 

0.10 

15.62       20.10 
18.50       19.86 

2.37 
2.44 

}       2.41 

2.41 

337.30 

0.20 

19.64       21.75 
20.80       21.22 

4.77 
4.83 

}       4.80 

4.84 

344.85 

0.25 

22.40       24.20 
20.90       21.90 

6.13 
5.98 

}       6.06 

6.08 

343.90 

0.30 

18.50       19.94 
16.82       18.42 

7.23 
7.23 

}       7.23 

7.20 

341.10 

0.40 

18.70       19.10 
20.80       21.20 

9.51 
9.72 

}       9.62 

9.65 

343.45 

0.50 

20.78       20.94 
20.02       20.90 

12.02 
12.17 

}     12.10 

12.10 

342.50 

0.60 

21.10       21.80 
23.70       24.80 

14.34 
14.57 

}     14.46 

14.63 

347.60 

0.70 

19.70       20.60 
23.20       25.10 

16.79 
17.02 

}     16.91 

17.03 

344.90 

0.80 

17.50       18.42 
19.20       20.20 

19.39 
19.54 

}     19.47 

19.20 

338.75 

0.891 

17.50       20.20 

21.19     !       21.19 

21.46 

346.50 

0.90 

19.10       20.20 

21.89     |       21.89 

21.71 

340.10 

1.00 

22.20       24.00 

24.80      ] 

" 

21.90       23.00 

24.39     1}     24.56 

24.35 

339.84 

it 

90  SO        91    QO 

94    Kf> 

£t\J  .  <J\J             —  1    .  -<\> 

—  t  .  •  >'  '       i 

Mean.  .  .  . 

341.41 

All  the  solutions  except  one  were  made  up  on  the  "weight-normal" 
basis — that  is,  by  dissolving  a  gram-molecular  weight  of  the  sugar,  or 
some  decimal  part  of  the  same,  in  1,000  grams  of  water. 

Table  10  gives,  for  Series  I,  the  weight-normal  concentrations  of 
the  solutions ;  the  extreme  temperature  of  the  bath  during  each  experi- 
ment; the  observed  osmotic  pressure;  the  mean  osmotic  pressure  for 
each  concentration;  the  mean  calculated  gas  pressures  of  the  solute 
if  its  volume  is  reduced  to  that  of  the  solvent',  and,  finally,  the  molecular 
weights  which  are  calculated  from  the  mean  osmotic  pressures  by  the 


116 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


formula  M  =  W 


22.488+0.0824* 


,  in  which  oxygen  is  assigned  an  atomic 


weight  of  16. 

The  use  of  0  =  16  as  a  standard  for  the  calculation  of  molecular 
weights  was  continued  only  through  the  first  series  of  measurements. 
In  this  series,  cane  sugar  is  considered  to  have  a  molecular  weight  of 
342.22.  In  all  later  work,  H  =  1  was  employed  as  the  standard,  and  the 
molecular  weight  of  sugar  is  339.60.  Accordingly,  the  formula  given 

,.         ,,     TJ, 22.265 +0.0817*  . 
above  becomes,  in  subsequent  computations,  M  =  W  -         —p —      — ,  in 

which  M  is  the  molecular  weight;  P  the  observed  osmotic  pressure;  W 
the  weight  of  the  substance  which  is  dissolved  in  1,000  grams  of  water; 
22.265  the  theoretical  pressure  (at  0°)  of  a  gram-molecular  weight  of 
a  gas  when  its  volume  is  1  liter;  and  0.0817  is  the  temperature  coefficient, 
either  of  a  gas  or  of  osmotic  pressure.  The  values  are  based  on  the 
weight  of  a  liter  of  hydrogen  as  given  by  Morley,  and  corrected  to  the 
latitude  and  altitude  of  the  place  of  work. 

TABLE  11. — Cane  sugar,  Series  I.     Variations  in  the  temperature  of  the  bath. 


Concentration. 

Variation. 

Concentration. 

Variation. 

Concentration. 

Variation. 

degrees. 

degrees. 

degrees. 

0.05 

0.88 

0.30 

1.44 

0.70 

0.90 

" 

0.70 

" 

1.60 

" 

1.90 

0.10 

4.48 

0.40 

0.40 

0.80 

0.92 

" 

1.36 

" 

0.40 

" 

1.00 

0.20 

2.11 

0.50 

0.16 

0.891 

2.70 

" 

0.42 

" 

0.88 

0.90 

1.10 

0.25 

1.80 

0.60 

0.70 

1.00 

1.80 

" 

1.00 

" 

1.10 

" 

1.10 

" 

1.10 

The  difference  between  the  highest  and  lowest  temperature  of  the  bath 
is  given  for  each  experiment  in  Table  11.  At  the  present  time,  when 
a  variation  of  0.05°  in  bath  temperature  is  regarded  as  vitiating  a  reading 
of  pressure,  these  differences  appear  appallingly  large. 

Judging  by  the  non-appearance  of  solute  in  the  water  surrounding 
the  cells,  and  the  ability  of  the  cells  to  sustain  considerable  pressure, 
it  was  concluded  that  the  membrane  had  not  leaked.  Other  possible 
sources  of  dilution — of  which  much  will  be  said  hereafter — did  not  at 
that  time  impress  us  as  likely  to  affect  materially  the  pressures  devel- 
oped in  the  cells.  It  was  suspected,  however,  in  the  beginning  that 
the  sugar  might  be  subject  to  some  inversion,  for  which  it  would  be 
necessary  to  correct  the  observed  pressures.  All  the  solutions,  when 
taken  from  the  cells,  were  therefore  examined  for  invert  sugar  by 
the  then  most  approved  form  of  Fehling's  method.  Evidence  of  its 
presence  was  found  in  all  the  solutions,  but  it  was  only  in  the  more 


CANE    SUGAR. 


117 


concentrated  of  them  that  the  amount  sufficed  for  even  an  approximate 
quantitative  estimation.  The  osmotic-pressure  correction  equivalents 
of  the  quantities  found  are  given  in  Table  12. 

TABLE  12. — Cane  sugar,  Series  I.     Correction  for  inversion  found  by  Fehling's  method. 


Concentration. 

Correction. 

Concentration. 

Correction. 

Concentration. 

Correction. 

atmos. 

atmos. 

atmos. 

0.05 

0.00 

0.30 

0.00 

0.70 

0.02 

" 

0.00 

" 

0.00 

" 

0.02 

0.10 

0.00 

0.40 

0.00 

0.80 

0.04 

" 

0.00 

" 

0.00 

" 

0.04 

0.20 

0.00 

0.50 

0.00 

0.891 

0.04 

" 

0.00 

" 

0.02 

0.90 

0.05 

0.25 

0.00 

o.co 

0.02 

1.00 

0.05 

" 

0.00 

" 

0.05 

" 

0.04 

« 

0.04 

The  quantities  given  in  the  table  are  not  the  full  osmotic  equivalents 
of  the  invert  sugar.  They  are  equal  to  one-half  the  pressure  which  is 
exerted  by  the  products  of  inversion,  since  that  is  the  proportion  which 
is  to  be  deducted  from  the  observed  pressures  in  correcting  sucrose 
for  the  presence  of  hexoses. 

A  very  noteworthy  feature  of  Series  I  was  the  close  agreement 
between  the  known  molecular  weight  of  cane  sugar  and  that  calculated 
from  the  observed  pressures,  on  the  presumption  that  the  osmotic 
pressure  of  solutions  obeys  the  laws  of  Gay-Lussac  and  Boyle.  It 
will  be  seen,  in  Table  10,  that  the  mean  molecular  weight  derived 
from  25  determinations,  on  13  different  concentrations  of  solution, 
was  341.41;  while  the  theoretical  value  (if  oxygen  =  16)  is  342.22. 
The  coincidence  is  better  expressed  in  the  form  of  the  ratio  of  osmotic 
to  theoretical  gas  pressure,  as  in  Table  13.  The  disagreement  appears 

TABLE  13. — Cane  sugar,  Series  I.    Ratio  of  osmotic  to  calculated  gas  pressure. 


Concen- 
tration. 

Osmotic 
pressure. 

Gas 

pressure. 

Ratio. 

Concen- 
tration. 

Osmotic 
pressure. 

Gas 
pressure. 

Ratio. 

0.05 
0.10 
0.20 
0.25 
0.30 
0.40 
0.50 

1.27 
2.41 
4.80 
6.06 
7.23 
9.62 
12.10 

1.21 
2.41 

4.84 
6.08 
7.20 
9.65 
12.10 

1.050 
1.000 
0.992 
0.997 
1.004 
0.997 
1.000 

0.60 
0.70 
0.80 
0.891 
0.90 
1.00 

14.46 
16.91 
19.47 
21.19 
21.89 
24.56 

14.63 
17.03 
19.20 
21.46 
21.71 
24.35 

Mean  .... 

0.989 
0.993 
1.001 
0.987 
1.008 
1.009 

1.002 

to  be  large  in  the  case  of  the  most  dilute  solution,  but  of  this  it  is  to  be 
said  that  an  experimental  error  of  0.1  atmosphere  in  the  measurement 
of  the  osmotic  pressure  of  the  0.05  weight-normal  solution  leads  to  an 
error  of  30.4  units  in  the  estimated  molecular  weight,  or  of  0.09  in 


118      OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

the  ratio  of  osmotic  to  gas  pressure.     In  all  other  concentrations  the 
ratio  approaches  unity. 

The  striking  agreement  between  the  observed  osmotic  and  theoretical 
gas  pressure,  which  is  seen  in  Tables  10  and  13,  gave  the  author,  for 
a  time,  much  more  confidence  in  the  trustworthiness  of  these  first 
results  than  they  were  afterwards  found  to  deserve.  The  evidence 
furnished  by  them  appeared  to  confirm  the  conclusions  of  van't  Hoff 
regarding  the  measurements  of  Pfeffer.  The  solutions  employed  were, 
in  general,  much  more  concentrated  than  those  of  Pfeffer,  and  more 
concentrated  also  than  those  to  which  van't  Hoff  restricted  his  de- 
ductions regarding  osmotic  pressure;  but  it  was  believed  that,  by  the 
adoption  of  the  ''weight-normal"  system,  the  term  6  in  the  van  der 
Waals  equation  had  been  practically  eliminated  for  osmotic  pressure. 
There  was,  therefore,  no  apparent  inconsistency  in  the  seeming  con- 
formity of  concentrated  as  well  as  dilute  solutions  to  the  formula  of 
van't  Hoff.  The  correctness  of  this  view  of  the  function  of  the  weight- 
normal  system  will  be  maintained  later  by  means  of  data  whose  validity 
can  not  be  questioned.  The  real  ground  for  suspecting  the  trust- 
worthiness of  the  results  of  Series  I  was  revealed  by  the  polariscope 
in  connection  with  the  work  of  Series  II. 

SERIES  II.* 

The  measurements  of  Series  II  were  made  under  more  favorable 
conditions  than  those  of  Series  I.  Some  of  the  improvements  which 
were  introduced  will  be  enumerated: 

1.  In  the  earlier  work  there  had  been  much  uncertainty  as  to  the 
exact  capacity  of  the  upper  end  of  the  manometer,  where  the  form 
of  the  tube  had  been  altered  in  closing  the  instrument  in  the  flame. 
The  original  calibration  could  not  hold  for  this  part  of  the  manometer, 
and  there  was  no  obvious  method  of  ascertaining  its  capacity  directly. 
It  had  been  customary,  therefore,  to  measure  the  height  of  the  affected 
part  and  to  assign  to  it  a  spherical,  or  a  conical,  or  a  conico-spherical 
form,  according  to  its  appearance.  The  diameter  of  the  bore  at  the 
base  was  known  from  the  calibration.  This  method  would  have 
sufficed  if  the  form  had  been  strictly  spherical  or  purely  conical;  but, 
as  a  rule,  it  was  neither  the  one  nor  the  other,  but  a  mixture  of  the 
two,  and  it  was  necessary,  in  estimating  capacity,  to  guess  in  what 
proportion  each  form  was  represented.  It  could  be  easily  proved,  by 
examples  of  the  effects  of  minute  errors  in  manometric  work,  that  the 
problem  was  one  of  great  importance.  It  was  solved  satisfactorily 
by  filling  the  upper  end  of  the  manometer  with  mercury  in  the  manner 
described  in  a  previous  chapter. 

*Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer,  E.  J.  Hoffman,  and  W.  L.  Kennon.  Am. 
Chem.  Jour.,  xxxvi,  39. 


CANE   SUGAR. 


119 


FIG.  47. — First  bath  in  which  water  and  air  were  circulated. 

(A)  Water  compartment;  (B)  air  compartment;  (a),  (p),  (g),  and  (r)  hair  packing;  (6)  tube 
through  which  the  water  in  (A)  was  pumped;  (/)  tube  through  which  air  in  CB)  was  pumped; 
(d)  and  (h)  propellers;  (0  and  (&)  friction  pulleys;  (t)  and  (;')  friction  disks;  (2)  and  (2)  holes 
for  escape  of  water  into  bath. 


120 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


Another  improvement  in  manometers  which  was  made  before 
beginning  Series  II  was  the  introduction  of  the  "safety  bulb,"  which 
is  blown  in  the  tube  just  below  the  calibrated  portion,  and  which 
prevents  an  escape  of  the  gas  when  it  is  under  diminished  pressure. 

The  methods  of  calibration 
were  also  improved,  and  more 
attention — although   by  no 
means  so  much  as  at  a  later      «« 
period  —  was  given  to  the  ir-      J1U 
regularities  of  capillary  depres- 
sion. 

2.  The  greatest  improvement 

in  apparatus,  however,  was  in  the 
devices  for  maintaining  temperature, 
though  gas  and  electric  stoves,  regu- 
lated by  thermostats,  were  not  intro- 
duced for  the  control  of  bath  and 
room  temperatures  until  later.  The 
first  bath  so  furnished  was  the  crude 
forerunner  of  that  seen  in  Figures  35, 
36,  and  37,  pages  69  and  70.  It  was 
a  large  rectangular  affair  (Figure  47) 
consisting  of  two  superimposed  com- 
partments. The  lower  one  contained 
water,  which  was  kept  in  circulation 
by  means  of  a  pump  (Figure  48). 
The  air  in  the  upper,  or  manometer, 
compartment  was  drawn  continu- 
ously through  pipes  (Figures  48  and 
49)  lying  in  the  water  below  by  means 
of  a  second  pump.  The  temperature 
of  the  water  in  which  the  cells  were 
suspended  was  regulated,  as  best  it 
could  be  at  that  time,  by  means  of 
immersed  electric  stoves,  which  were 
controlled  by  a  thermostat ;  while  that 
of  the  air  in  the  manometer  space  was 
kept  approximately  the  same  by  pass- 
ing it  uninterruptedly  through  the  pipes  in  the  water.  The  walls  were 
all  double  and  were  packed  with  hair. 

It  will  be  shown  later  that  a  system  of  bath  regulation  such  as  that 
described  is  exceedingly  imperfect.  Nevertheless,  it  was  a  great 
improvement  on  that  employed  in  Series  I. 

3.  The  improvement  in  the  cathetometer  (Figure  25,  page  44)  which 
enabled  us  to  dispense  with  the  micrometer  eye-piece  of  the  telescope, 


FIG.  48. — Pumping  arrangements   on  larger 
scale  than  in  Figure  47. 


CANE    SUGAR. 


121 


and  which  remedied  the  "lurching"  effects  of  the  older  method  of 
adjustment,  was  introduced. 

4.  The  addition  to  our  equipment  which  gave  the  most  satisfaction, 
and  which  proved  to  be  the  most  indispensable  of  all  our  instruments— 
if  comparisons  are  legitimate  in  a  work  whose  success  depends  on  the 
perfection  of  every  one  of  a  multitude  of  conditions — was  a  Schimdt 
and  Hansch  saccharimeter  of  the  best  construction. 

It  is  to  be  remembered  in  this  connection  that,  in  Series  I,  we  had 
no  means  of  ascertaining  what  had  occurred  in  the  solutions  while 
in  the  cells  except  the  test  of  Fehling  and  the  examination  of  the 
solvent  in  which  the  porous  part  of  the  cells  was  immersed;  also  that, 
having  found  no  solute  in  the  latter,  and  but  little  invert  sugar  by  the 
former,  we  were  obliged  to  conclude,  from  the  evidence  available,  that 
the  solutions  had  maintained  their  concentration  without  much  altera- 


FIG.  49. — Interior  view  of  water  compartment  with  covers  partly  removed. 
(e)  and  (e')  air  tubes;  (6)  tube  for  circulating  water.     No  devices  for  heating  or  cooling  the  water. 

tion  of  the  solute.  If  this  conclusion  were  correct,  and  it  was  believed 
to  be  so  in  the  main,  the  results  of  the  measurements  of  Series  I  were 
trustworthy  and  furnished  strong  experimental  evidence  in  support 
of  the  deductions  of  van't  Hoff. 

It  had  been  suspected,  however,  that  the  inversion  occurring  in  the 
cells  was  somewhat  larger  than  it  had  been  found  to  be  by  the  method 
of  Fehling,  and  the  object  sought  by  the  introduction  of  the  polariscope 
into  the  investigation  was  to  measure  this  supposed  greater  inversion 
by  the  more  accurate  optical  method.  The  quantity  of  invert  sugar 
was  to  be  measured  by  the  loss  in  rotation,  and  one-half  the  pressure- 
equivalent  of  the  invert  sugar  so  found  was  to  be  deducted  from  the 
observed  pressure,  in  order  to  arrive  at  the  correct  osmotic  pressure  of 
the  original  solution  of  cane  sugar. 


122 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


The  loss  in  rotation  was  ascertained  to  be  very  much  larger  than  had 
been  anticipated.  For  the  time  being,  however,  it  was  all  ascribed  to 
inversion,  notwithstanding  certain  suspicions  which  will  be  discussed 
a  little  later.  Accordingly,  in  a  paper  which  was  published  soon  after 
the  completion  of  Series  II,  corrections  for  inversion  were  applied  to 
the  observed  pressures,  which  were  proportional  and  equivalent  to  the 
losses  in  rotation. 

The  uncorrected  pressures  of  this  series  are  given  in  Table  14: 

TABLE  14. — Cane  Sugar,  Series  II.     Extreme  temperatures  of  the  bath;  loss  in 
rotation;  observed  osmotic  pressures;  calculated  gas  pressures. 


Concentration. 

Temperature. 

Loss  in 
rotation. 

Observed 
osmotic 
pressure. 

Gas 
pressure. 

Ratios. 

degrees. 

degrees. 

0.1 

24.  00  to  24.  10 
24.15       24.25 

0.50 
0.50 

2.58 
2.62 

2.41 

2.42 

}     1.077 

0.2 

20.00       20.95 

0.70 

4.75 

4.79 

1 

" 

20.90       21.35 

0.70 

4.82 

4.80 

\     1.003 

" 

21.50       21.85 

0.50 

4.88 

4.81 

1 

0.3 

19.65       20.10 

0.60 

7.28 

7.16 

)i   m  x. 

" 

21.55       21.70 

0.50 

7.31 

7.21 

i  .  U1O 

0.4 

21.45       21.75 

0.80 

9.76 

9.61 

I     1.012 

" 

22.10       22.20 

1.30 

9.71 

9.63 

0.5 

22.50       22.70 
23.70       23.70 

1.40 
1.20 

12.28 
12.41 

12.06 
12.10 

}      1.022 

0.6 

24.30       24.40 

2.80 

14.82 

14.55 

} 

" 

24.15       24.30 

1.90 

15.00 

14.55 

[     1.028 

" 

24.10       24.10 

1.80 

15.06 

14.54 

j 

0.7 

23.35       24.00 

2.60 

17.38 

16.94 

I       1    noc 

" 

23.74       24.30 

2.20 

17.32 

16.93      /     *•"'" 

0.8 

23.57       23.60 

3.20 

19.83 

19.35      1     t   noo 

" 

23.65       23.70 

3.90 

19.77 

19.36 

r       j.  .  \j£,tt 

0.9 

24.65       24.90 

2.50 

22.25 

21.86 

i     non 

" 

24.65       24.90 

2.40 

22.32 

21.86      /     i-u'iu 

1.0 

23.55       23.60 

2.40 

24.83 

24.19 

i 

24.50       24.60 

4.00 

24.78 

24.28 

|      1  .  024 

Total 

=  38.40  =  9.37  atmospheres.  Mean  =  1.025 

No  attempt  was  made  in  Series  II  to  keep  the  bath  at  a  particular 
temperature  throughout.  It  was  endeavored  simply  to  maintain  it 
through  each  experiment  at  whatever  temperature  the  solution  was 
found  to  have  when  the  cell  was  filled  and  closed — both  solution  and 
cell  having  stood  for  some  time  previously  in  the  bath. 

That  the  fluctuations  in  Series  II  were  much  smaller  than  in  Series  I 
will  be  seen  in  Table  15. 

The  most  surprising,  and  at  the  same  time  the  most  perplexing, 
feature  of  the  results  was  the  large  loss  in  rotation.  It  amounted,  as 
will  be  seen  by  Table  14,  to  a  total  of  38.4°,  which  was  equivalent 
to  about  9.37  atmospheres  of  osmotic  pressure.  Expressed  in  another 
way,  the  total  loss  in  rotation  amounted  to  2.86  per  cent  of  the  sum 
of  all  the  original  rotations  of  the  solutions  whose  pressure  had  been 


CANE    SUGAR. 


123 


determined.  We  were  strongly  inclined  to  ascribe  it  in  the  main,  if 
not  altogether,  to  inversion.  But  why  should  so  much  inversion  occur  in 
Series  II  when  so  little  of  it  had  been  detected  in  Series  I  by  the  method 
of  Fehling?  Admitting  the  greater  accuracy  of  the  optical  method,  it  was 
not  possible  that  so  much  invert  sugar  should  have  escaped  detection  in 
Series  I,  especially  since  the  fault  of  Fehling's  method  is  its  liability  to 
overestimate  rather  than  underestimate  the  products  of  inversion. 

TABLE  15. — Cane  sugar,  Series  I  and  II.    Extreme  variations  in  bath  temperature. 


Concentration. 

Series  I. 

Series  II. 

Concentration. 

Series  I. 

Series  II. 

degrees. 

degree. 

degrees. 

degree. 

0.1 

4.48 

0.10 

0.6 

0.70 

0.10 

" 

1.36 

0.10 

" 

1.10 

0.15 

0.2 

2.11 

0.95 

" 

0.00 

" 

0.42 

0.45 

0.7 

0.90 

0.65 

" 

0.35 

" 

1.90 

0.56 

0.3 

i!44 

0.45 

0.8 

0.92 

0.03 

11 

1.60 

0.15 

" 

1.00 

0.05 

0.4 

0.40 

0.30 

0.9 

2.70 

0.25 

" 

0.40 

0.10 

" 

1.10 

0.25 

0.5 

0.16 

0.20 

1.0 

1.80 

0.05 

" 

0.88 

0.00 

" 

1.10 

0.10 

" 

1.10 

Means  = 

1.31 

0.24 

The  explanation  which  sufficed  for  a  time  was  plausible.  It  was 
at  the  beginning  of  the  work  in  Series  II  that  the  penicillium  pest 
made  its  appearance,  and  it  was  reasonable,  as  we  then  thought,  to 
ascribe  the  apparently  greater  inversion  in  Series  II  to  an  infection 
of  the  membranes  by  penicillium.  The  mistake  in  practice  which  was 
made  was,  of  course,  in  wholly  discontinuing  for  a  time  the  use  of 
Fehling's  test  as  soon  as  the  polariscope  became  available.  An  exami- 
nation of  the  solutions  of  Series  II  by  both  methods  would  have  shown 
at  once  the  inconsistency  of  our  interpretation  of  the  loss  in  rotation. 
It  was  probably  true,  however,  that  the  penicillium  did  cause  some 
inversion  in  the  solutions  of  Series  II,  though  by  no  means  enough  to 
account  for  the  whole,  or  any  larger  part,  of  the  loss  in  rotation;  for 
it  was  found  in  later  series — where  the  Fehling  test  was  again  applied, 
and  after  the  solutions  and  cells  had  been  habitually  treated  with 
all  the  care  necessary  for  the  suppression  of  penicillium — that  the 
evidences  of  inversion  had  nearly  disappeared. 

A  circumstance  which  at  the  time  tended  to  strengthen  the  impres- 
sion that  the  loss  in  rotation  was  due  to  inversion  was  the  molecular 
weight  which  was  derived  from  the  osmotic  pressures  after  correcting 
them  for  inversion  proportional  to  the  observed  losses.  The  mean 
molecular  weight  thus  obtained  was  337.59  (H  =  1)  instead  of  339.60,  the 
theoretical  value.  The  mean  molecular  weight  derived  from  Series  I 
was  341.41  (0  =  16)  instead  of  342.22. 


124      OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

The  seeming  adequacy  of  the  interpretation  of  the  loss  in  rotation 
which  is  given  above,  and  the  attractive  concordance  which  the  results 
of  Series  I  and  II  acquired  through  its  application  were  afterwards 
proved  to  be  wholly  illusive. 

It  was  realized  from  the  beginning  that  the  diminished  rotation 
could  also  be  produced  by  dilution.  Indeed,  this  would  have  been  the 
most  obvious  interpretation  of  the  phenomenon  if  the  membranes  had 
failed  to  retain  perfectly  the  solute.  But,  in  the  absence  of  leakage, 
it  was  difficult  to  explain,  as  due  to  dilution,  a  loss  in  rotation  which 
amounted  to  an  average  of  2.86  per  cent,  or  to  an  average  surreptitious 
introduction  into  the  cells  of  nearly  0.5  cubic  centimeter  of  the  solvent. 

There  were,  nevertheless,  three  sources  of  dilution  which  were  apparent 
enough,  but  it  was  not  believed  that  these  could  account  for  more  than 
a  small  fraction  of  the  loss.  However  much  their  aggregate  effect  may 
have  been  underestimated  in  the  beginning,  they  were  not  at  any  time 
ignored  or  neglected.  It  was  recognized  that,  in  order  to  settle  definitely 
the  question  of  loss  in  rotation,  all  sources  of  dilution  must  be  sup- 
pressed by  improvement  in  the  method  and  in  the  manipulation. 

In  discussing  the  three  obvious  sources  of  dilution  which  have  been 
referred  to,  it  will  be  necessary  to  introduce  observations  and  facts 
which  belong  to  later  periods  in  the  history  of  the  investigation.  If 
this  is  not  done,  it  will  be  difficult  to  place  the  results  of  Series  II  in  the 
light  in  which  the  author  now  sees  them. 

1.  It  has  already  been  intimated  that  the  closing  of  the  cell  was  a 
difficult  performance  which  required  considerable  time — in  the  begin- 
ning, about  15  minutes.  During  the  whole  operation,  the  cell  contents 
were  under  a  pressure  which  was  less  than  the  true  osmotic  pressure 
of  the  solution.  Throughout  the  whole  of  the  closing  period,  therefore, 
the  solutions  were  undergoing  a  dilution  by  solvent  taken  in  through 
the  membranes.  If  the  impression  is  correct  that  the  rate  at  which 
the  solvent  is  taken  in,  under  such  conditions,  is  proportional  to  the 
difference  between  the  existing  and  the  true  osmotic  pressure  of  the 
solution,  the  amount  of  dilution  accomplished  during  the  closing  period 
must  have  varied  considerably  from  cell  to  cell.  Operator  "No.  1" 
endeavored  to  maintain  the  highest  possible  "existing  pressure"  through- 
out the  operation,  but  was  never  able  to  equal  the  osmotic  pressure. 
Moreover,  the  pressure  was  constantly  fluctuating  in  consequence  of 
the  manipulations  of  "No.  2"  with  the  "fang." 

It  was  evident  that,  in  order  to  suppress  this  initial  dilution  of  the 
cell  contents,  "No.  1"  and  "No.  2"  must  cooperate  in  such  a  way  as  to 
maintain  the  highest  possible  pressure  upon  the  solution  throughout 
the  closing  period;  also,  and  above  all,  that  the  time  required  for  closing 
must  be  greatly  shortened.  The  first  improvement  in  the  latter  direc- 
tion was  accomplished  by  tightly  wrapping  and  tying  the  lower  end  of 
the  rubber  stopper — just  above  the  enlargement  on  the  manometer — 


CANE   SUGAR.  125 

with  twisted  shoemakers'  thread.  The  lower  end  of  the  stopper,  whose 
introduction  through  the  constricted  mouth  of  the  glass  tube  had  pre- 
viously been  so  slow  and  difficult,  was  thus  made  much  smaller.  The 
effect  of  the  improvement  was  to  reduce,  by  more  than  one-half,  the 
time  required  for  closing  the  cells.  It  was  still  further  reduced  by 
gradual  improvement  in  the  cooperative  manipulation  of  "No.  1"  and 
"No.  2"  until  finally  a  cell  could  be  closed  in  less  than  one-fifth  of  the 
time  which  was  required  in  the  beginning.  The  effect  of  rapid  and 
judicious  manipulation  in  diminishing  the  total  loss  in  rotation  was 
so  marked  that  "quick  closing"  soon  became,  and  continued  to  be,  one 
of  the  principal  items  in  all  schemes  for  the  improvement  of  the  method. 

Another  method  of  diminishing  initial  dilution,  which  was  resorted 
to  in  the  latter  half  of  the  work,  consisted  in  dipping  the  cells — after 
filling  and  before  closing  them — in  a  solution  of  sugar.  The  concentra- 
tion of  the  solutions  so  employed  was  at  first  equal  to  that  of  the  solu- 
tions in  the  cells.  Afterward  they  were  made  more  concentrated.  The 
purpose  of  the  dipping  process  was,  of  course,  to  force  the  solvent 
which  filled  the  porous  wall  outside  the  membrane  to  distribute  itself 
between  the  solution  within  the  cell  and  that  upon  the  exterior  surf  ace. 
The  solution  upon  the  outside  of  the  cell  was  afterward  removed  as 
completely  as  possible  by  rinsing,  and  by  soaking  the  cell,  before  locat- 
ing it  finally  in  the  bath,  in  fresh  water  which  was  repeatedly  renewed. 
The  diminution  in  the  total  loss  in  rotation  which  followed  the  introduc- 
tion of  the  custom  of  "dipping"  was  also  considerable. 

The  combined  effect  of  shortening  the  time  required  for  closing  the 
cells,  and  of  the  process  of  dipping  them,  upon  the  total  loss  in  rotation 
sufficed  to  prove  that  considerable  dilution  must  have  occurred  at  this 
period  in  the  case  of  the  earlier  series. 

2.  The  practice  of  wrapping  and  tying  with  twisted  and  waxed  shoe- 
makers' thread  all  that  portion  of  the  rubber  stopper  which  remained 
outside  the  glass  tube  was  followed  from  the  beginning.  The  object 
was  to  confine  the  exposed  part  of  the  stopper  within  a  rigid  shell,  so 
that  none  of  the  rubber  within  the  glass  tube  could  be  forced  out  of  it 
under  pressure.  This  seemingly  simple  operation  proved  to  be  exceed- 
ingly difficult.  In  fact,  it  was  performed  with  perfect  success  only 
in  the  last  four  of  the  eight  preliminary  series  of  measurements.  The 
upper  part  of  the  stopper  suffered,  despite  the  careful  winding,  con- 
siderable distortion  through  a  forcing  out  of  some  of  the  rubber  between 
the  successive  turns  of  the  thread.  All  such  displacements  of  material 
represented,  of  course,  an  equivalent  enlargement  of  the  capacity  of  the 
cells  and  a  corresponding  dilution  of  the  solutions.  A  phenonenon 
which  always  attended  a  distortion  of  the  stopper  was  an  upward  dis- 
placement of  the  manometer  while  the  cell  was  in  the  bath.  Such  dis- 
placements were  recorded  in  the  second  and  succeeding  series  and  were 
regarded  as  a  test  of  some  value  of  the  progress  which  had  been  made  in 


126 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


the  effort  to  secure  a  cell  of  fixed  capacity.     The  upward  displacements 
of  the  manometer  in  Series  II  are  given,  as  an  illustration,  in  Table  16. 

TABLE  16. — Cane  sugar,  Series  II.     Upward  displacements  of  the  manometers  (mm.). 


Concen- 
tration. 

Displace- 
ment. 

Concen- 
tration. 

Displace- 
ment. 

Concen- 
tration. 

Displace- 
ment. 

0.1 

0.07 

0.4 

0.30 

0.7 

0.24 

" 

0.54 

" 

1.18 

" 

0.07 

0.2 

2.78 

0.5 

0.16 

0.8 

0.32 

" 

1.44 

" 

0.32 

" 

1.80 

" 

0.19 

0.6 

3.78 

0.9 

1.21 

0.3 

0.12 

" 

0.83 

1.0 

0.88 

0.10 

0.64 

2.97 

In  Series  I  there  was  much  actual  slipping  of  the  manometer  in  the 
rubber  stopper,  in  consequence  of  which  the  enlarged  part  of  the  manom- 
eter— the  bulb  blown  near  the  end — was  frequently  forced  out  of  sight 
into  the  stopper.  It  usually  stopped,  in  such  cases,  just  below  the 
constricted  mouth  of  the  glass  tube.  The  principal  purpose  of  the  con- 
stricted mouth  of  the  tube  and  of  the  enlargement  on  the  end  of  the 
manometer  was,  originally,  to  prevent  the  instrument  from  being 
pushed  out  of  the  cell.  The  slipping  of  the  manometer  in  the  stopper 
was  remedied  by  wrapping  and  tying  the  lower  end  of  the  latter  in  the 
manner  already  described. 

The  upward  displacement  of  the  manometer  disappeared  after  the 
fourth  series.  There  was  therefore  some  dilution  in  the  first  four  series, 
which  was  due  to  an  enlargement  of  the  capacity  of  the  cells. 

3.  The  opening  of  the  cell,  after  a  measurement,  like  the  closing  of  it, 
was  originally  a  process  which  required  about  15  minutes.  During 
this  time,  the  contents  of  the  cell  were  again  under  a  pressure  which  was 
less  than  the  osmotic  pressure,  and  dilution  of  the  solution  necessarily 
ensued.  For  reasons  which  will  be  given  hereafter,  dilution  occurring 
at  this  period  was  a  much  more  serious  matter  than  that  which  took 
place  during  the  closing  of  the  cells  or  through  an  increase  in  their  capa- 
city under  pressure.  It  was  necessary,  therefore,  to  suppress  it  as  expe- 
ditiously  as  possible.  Several  remedial  measures  were  resorted  to :  (1) 
Every  effort  was  made  to  increase  the  rapidity  of  the  necessary  manip- 
ulation; (2)  the  rubber  stopper  was  pierced  with  a  hollow  needle  before 
attempting  to  withdraw  it;  (3)  "dipping"  the  .cells  before  opening  them 
was  practiced ;  (4)  a  method  was  devised  for  slitting  the  stopper  through- 
out its  whole  length,  which  made  it  possible  eventually  to  remove  the 
manometer  in  less  than  a  minute,  and  without  reducing  the  pressure  upon 
the  cell  contents  below  that  of  the  atmosphere. 

The  final  result  of  all  the  measures  taken  to  eliminate  the  three 
known  sources  of  dilution  was  the  complete  disappearance  of  loss  in  rota- 


CANE    SUGAR.  127 

Hon.  In  other  words,  it  was  proved  at  last  that  the  observed  loss  in 
rotation  was  due  to  dilution  and  not  to  inversion. 

Having  found  that  the  uncertainty  regarding  the  osmotic  pressures  of 
the  solutions  was  due  to  dilution,  and  knowing  the  extent  of  the  dilution, 
the  question  arises  whether  it  is  legitimate  to  correct  the  observed  pres- 
sures of  Series  II  for  dilution  as  they  were  originally  corrected  for  inversion. 
The  author  is  of  the  opinion  that,  with  certain  reservations,  this  may  be 
done,  and  that  the  results  will  thereby  acquire  a  new  standing  in  the  his- 
tory of  the  investigation  which  is  more  in  accordance  with  their  merits. 

The  absolute  futility  of  attempting  to  correct  observed  pressures  for 
dilution  which  is  due  to  leakage  of  the  membranes  has  been  emphasized 
in  another  chapter.  The  reason  given  was  that  one  has  no  means 
of  ascertaining  the  magnitude  of  the  counter  pressure  which  is  exerted  by 
the  escaped  solute,  even  when  one  knows  how  much  of  it  has  passed 
through  the  membrane,  since  the  lost  material  does  not  distribute  itself 
quickly  and  uniformly  throughout  the  whole  body  of  solvent  which  is 
exterior  to  the  membrane,  but  remains,  for  the  greater  part,  in  the  pores 
of  the  cell,  giving  a  solution  next  to  the  membrane  whose  concentration 
is  unknown  and  can  not  be  determined. 

The  dilution  in  the  case  under  consideration  was  effected  without 
the  loss  of  solute,  and  solely  through  the  acquisition  of  solvent  at  three 
different  periods.  Moreover,  the  concentration  of  the  solutions  was 
determined  by  the  polariscope  after  the  dilution  had  ceased.  There 
can  be  no  question  as  to  the  propriety  of  correcting  for  the  dilution 
which  occurred  during  the  closing  of  the  cells  or  for  that  which  occurred 
while  they  were  in  the  bath,  since  both  were  finished  before  the  obser- 
vations on  the  osmotic  pressures  of  the  solutions  were  taken.  The 
dilution  of  the  third  or  opening  period  is  of  a  different  order,  in  that 
it  occurred  after  the  measurements  of  pressure  and  previous  to  the 
determinations  of  concentration  by  the  polariscope.  It  had  not,  there- 
fore, affected  the  pressures  in  the  cells,  and  it  should  be  deducted  from 
the  total  before  correcting  the  observed  pressures  for  dilution.  There 
are,  however,  no  means  of  ascertaining  how  much  of  the  known  total 
dilution  occurred  during  the  opening  of  the  cells.  It  is  probable, 
therefore,  that  pressures  which  are  corrected  for  the  total  dilution  will 
be  slightly  over-corrected.  The  excess  can  not  be  large,  because  the 
dilution  occurring  when  the  cells  were  opened  was  brought  under 
control  quite  early  in  the  investigation. 

With  this  intimation  that  the  results  are  somewhat,  but  probably 
not  largely,  overcorrected,  the  observed  pressures  of  Series  II,  which 
are  recorded  in  Table  14,  are  given  in  Table  17  as  corrected  for  dilution 
instead  of  inversion,  yet  no  higher  degree  of  accuracy  can  be  claimed  for 
these  corrected  pressures;  the  uncertainty  pertaining  to  the  correction 
for  dilution,  the  large  thermometer  effects  which  must  have  followed 
the  considerable  fluctuations  in  bath  temperature,  and  the  generally 


128 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


undeveloped  state  of  the  method  at  the  time,  all  conspire  to  diminish 
confidence  in  their  precision.  Nevertheless,  they  are  doubtless  much 
more  nearly  correct  than  were  the  values  obtained  by  correcting  the 
observed  pressures  for  inversion.  The  ratios  of  osmotic  to  gas  pressure 
are  all  considerably  above  unity  and  are  somewhat  irregular,  which 
suggests — but  does  not  prove — that  osmotic  pressure  does  not  con- 
form to  the  laws  of  Gay-Lussac  and  Boyle  for  gases.  In  a  rough  way, 
the  corrected  pressures  in  Table  17  approach  those  which  were  obtained 
later,  after  the  method  had  been  perfected;  and  they  foreshadow  much 
that  was  afterwards  found  to  be  true  upon  evidence  which  can  not  be 
questioned.  One  of  the  more  obvious  conclusions  to  be  drawn  from 
them — if  they  are  credited  with  approximate  accuracy — is  that  the 
excellent  molecular  weights  derived  from  Series  I,  and  also  from  Series 
II,  by  ascribing  all  loss  in  rotation  to  inversion,  were  entirely  fallacious. 

TABLE  17. — Cane  sugar,  Series  77.    Observed  osmotic  pressures  corrected  for  dilution 
and  the  ratios  of  osmotic  to  the  calculated  gas  pressures  of  the  solute. 


Concen- 
tration. 

Observed 
osmotic 
pressure. 

Corrected 
osmotic 
pressure. 

Ratio. 

Mean 
ratio. 

0.1 

2.58 
2.62 

2.69 
2.73 

1.114 
1.124 

}     1.119 

0.2 

4.75 

4.89 

1.021 

j 

" 

4.82 

4.96 

1.033 

f     1.025 

" 

4.88 

4.98 

1.021 

1 

0.3 

7.28 
7.31 

7.38 
7.43 

1.031 
1.031 

|     1.031 

0.4 

9.76 
9.71 

9.92 
9.98 

1.032 
1.036 

|     1.034 

0.5 

12.28 
12.41 

12.58 
12.67 

1.043 
1.047 

j     1.045 

0.6 

14.82 

15.44 

1.061 

] 

" 

15.00 

15.42 

1.060 

[     1.061 

" 

15.06 

15.46 

1.063 

j 

0.7 

17.38 
17.32 

17.96 
17.81 

1.060 
1.052 

|     1.056 

0.8 

19.83 
19.77 

20.57 
20.68 

1.063 
1.068 

J     1.066 

0.9 

22.25 
22.32 

22.83 
22.67 

1.044 
1.037 

|     1.041 

1.0 

24.83 

24.78 

25.43 
25.74 

1.051 
1.060 

j     1.056 

SERIES  III.* 

It  has  already  been  stated  that  in  Series  I  and  II  no  effort  was  made 
to  maintain  specific  temperatures  throughout — that  it  was  merely 
sought  to  keep  as  constant  as  possible,  for  the  time  being,  whatever 
temperature  the  bath  might  have  at  the  beginning  of  each  experiment. 
This  was  a  necessary  course  as  long  as  the  means  of  regulating  tem- 
perature continued  to  be  crude  and  to  a  high  degree  ineffective. 

*Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer,  and  W.  W.  Holland.  Am.  Chem.  Jour., 
xxxvii,  425. 


CANE    SUGAR. 


129 


In  Series  III,  on  the  other  hand,  an  attempt  was  made  to  maintain 
a  specific  temperature,  namely,  that  of  melting  ice.  It  was  not  entirely 
successful,  but  the  fluctuations  were  much  smaller  than  in  Series  I  and 
II.  The  bath  which  was  employed  was  the  large  rectangular  one  pre- 
viously described.  To  prepare  it  for  use  in  Series  III,  all  the  machinery 
was  removed  except  that  concerned  in  the  circulation  of  the  water, 
and  all  the  space  in  both  compartments,  except  that  actually  required 
for  the  cells  and  manometers,  was  filled  with  crates  for  the  storage  of  ice, 

TABLE  18. — Cane  sugar,  Series  HI.     Temperatures  of  bath;  loss  in  rotation;  observed  osmotic 
pressures;  and  calculated  gas  pressures  of  the  solute. 


Concentration. 

Temperature. 

Loss  in 
rotation. 

Observed 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

degrees. 

degrees. 

0.1 

0.18  to  0.36 

0.20 

2.45 

2.23 

I 

" 

0.14      0.26 

0.10 

2.45 

" 

1 

" 

0.14       0.38 

0.05 

2.37 

" 

| 

" 

0.14       0.38 

0.10 

2.39 

" 

J 

0.2 

0.16       0.26 
0.28       0.31 

0.15 
0.15 

4.78 
4.77 

4.46 

j     1.071 

0.3 

0.18       0.34 
0.14       0.18 

0.50 
0.60 

7.09 
7.11 

6.69 
6.68 

|     1.061 

0.4 

0.16       0.22 

0.55 

9.37 

8.91 

| 

" 

0.22       0.33 

0.60 

9.34 

8.92 

I     i   r\AQ. 

" 

0.26       0.34 

0.40 

9.36 

" 

f       X  .  U^o 

" 

0.12       0.16 

0.40 

9.31 

8.91 

J 

0.5 

0.14       0.38 

0.90 

11.66 

11.14 

" 

0.20       0.26 

1.00 

11.73 

44 

1  1  054 

" 

0.12       0.18 

1.05 

11.89 

14 

1.054 

" 

0.16       0.28 

0.75 

11.79 

41 

J 

0.6 

0.16       0.28 
0.20       0.25 

1.20 
1.30 

14.12 
14.11 

13.37 

|     1.056 

0.7 

0.16      0.32 
0.14      0.16 

1.20 
1.10 

16.65 
16.71 

15.60 

|     1.069 

0.8 

0.16      0.26 
0.16      0.26 

1.55 
1.60 

19.16 
19.13 

17.82 

|     1.075 

0.9 

0.30      0.33 
0.15       0.25 

1.85 
1.95 

21.92 
21.86 

20.06 
20.05 

|     1.091 

1.0 

0.16       0.30 

2.90 

24.53 

22.28 

44 

0.20       0.34 

3.50 

24.54 

" 

}     1.092 

" 

0.26      0.26 

2.00 

24.27 

22.29 

J 

Sum  = 

=  27.65  =  6.75  atmospheres.     Mean  =1.070 

which,  when  full,  contained  about  150  kilograms.  The  water  in  which 
the  ice  in  the  lower  half  of  the  crates  was  immersed  was  kept  in  circu- 
lation in  the  usual  manner,  and  its  level  was  maintained  by  means  of 
an  automatic  siphon.  It  was  hoped  to  secure,  by  this  arrangement,  a 
temperature  very  close  to  0°,  but  the  table  will  show  that  the  temper- 
atures actually  maintained  were  all  higher  than  that. 

The  fluctuations  in  bath  temperature  were  much  smaller  in  Series  III 
than  in  Series  II.     This,  however,  does  not  prove  that  any  progress  had 


130 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


been  made  in  the  general  improvement  of  the  facilities  for  the  main- 
tenance of  temperature;  since  it  is  easier,  by  means  of  circulating  ice 
water,  to  maintain  a  temperature  near  0°  than  to  secure  a  f air  degree 
of  constancy  by  means  of  regulating  devices  at  any  higher  temperature. 
That  some  progress  had  been  made  in  the  direction  of  securing 
constant  cell  capacity  is  shown  in  Table  19,  in  which  the  two  series  are 
compared  with  respect  to  the  upward  displacement  of  the  manometers. 

TABLE  19. — Cane  sugar,  Series  II  and  III.     Upward  displacements  of  the  manometers  (mm.) 


Concentration. 

Series  II. 

Series  III. 

Concentration. 

Series  II. 

Series  III. 

0.1 

0.07 

0.07 

0.5 

0.16 

0.24 

" 

0.54 

0.07 

" 

0.32 

0.22 

" 

0.09 

" 

0.07 

" 

0.09 

" 

0.38 

0.2 

2.78 

0.05 

0.6 

3.78 

0.48 

" 

1.44 

0.05 

" 

0.83 

0.10 

" 

0.19 

" 

0.64 

0.3 

0.12 

0.06 

0.7 

0.24 

0.61 

" 

0.10 

0.06 

" 

0.07 

0.45 

0.4 

0.30 

0.08 

0.8 

0.32 

0.01 

" 

1.18 

0.07 

" 

1.80 

0.34 

" 

0.22 

0.9 

1.21 

0.44 

" 

0.04 

" 

0.05 

1.0 

0.88 

0  20 

Average  upward  displacements: 

2.97 

0.59 

Series  11  =  0.94  mm.  .Series  111,0.22  mm 

0.91 

TABLE  20. — Cane  sugar,  Series  II  and  III.    Losses  in  rotation. 


Concentration. 

Series  II. 

Series  III. 

Concentration. 

Series  II. 

Series  III. 

degrees. 

degrees. 

degrees. 

degrees. 

0.1 

0.50 

0.20 

0.6 

2.80 

1.20 

" 

0.50 

0.10 

" 

1.90 

1.30 

" 

0.05 

" 

1.80 

" 

0.10 

0.7 

2.60 

1.20 

0.2 

0.70 

0.15 

" 

2.20 

1.10 

" 

0.70 

0.15 

0.8 

3.20 

1.55 

" 

0.50 

" 

3.90 

1.60 

0.3 

0.50 

0.50 

0.9 

2.50 

1.85 

" 

0.60 

0.60 

" 

2.40 

1.95 

0.4 

0.80 

0.55 

1.0 

2.40 

2.90 

" 

1.30 

0.60 

" 

4.00 

3.50 

" 

0.40 

" 

2.00 

(( 

040 

0.5 

1.40 

.  ^\j 
0.90 

Totals  

38.40 

27.65 

1.20 

1.00 

Per  cent  

2.86 

1.73 

" 

1.05 

Pressure  

9.37 

6.75 

0.75 

Further  evidence  of  progress  in  the  improvement  of  the  method  is 
to  be  found  in  Table  20,  in  which  the  losses  in  rotation  of  Series  II 
and  III  are  compared. 

The  evidence  presented  in  Table  20  relates  to  the  progress  which 
had  been  made  in  the  effort  to  suppress  dilution  from  any  or  all  sources, 


CANE   SUGAR. 


131 


while  that  in  Table  19  bears  upon  one  particular  source  of  dilution. 
The  reduction  of  the  total  loss  in  rotation  from  38.40°  in  22  determina- 
tions to  27.65°  in  27  experiments,  signified  considerable  improvement, 
especially  in  manipulation.  Expressed  in  pressure,  the  loss  was  reduced 
from  9.37  atmospheres  in  Series  II  to  6.75  in  Series  III.  The  com- 
parison is  better  made  by  means  of  percentages.  The  sum  of  all 
rotations  of  the  solutions  of  Series  II  was  1342.30°  and  the  sum  of  all  the 
losses  was  38.40°,  or  2.86  per  cent.  The  corresponding  numbers  for 
Series  III  were  1598.27°,  27.65°,  and  1.73  per  cent. 

TABLE  21. — Cane  sugar,  Series  III.    Observed  osmotic  pressures  corrected  for  dilution,  and  the 
ratios  of  osmotic  to  calculated  gas  pressure  of  the  solute. 


Concen- 
tration. 

Observed 
osmotic 
pressure. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

Mean 
ratio. 

0.1 

2.45 

2.49 

2.23 

1.117 

] 

" 

2.45 

2.47 

" 

1.108 

L     i   no** 

" 

2.37 

2.38 

" 

1.067 

/          1  .  U»7O 

" 

2.39 

2.41 

" 

1.081 

! 

0.2 

4.78 
4.77 

4.81 
4.80 

4.46 

1.078 
1.076 

}     1.077 

0.3 

7.09 
7.11 

7.19 
7.13 

6.69 
6.68 

1.076 
1.067 

}     1.071 

0.4 

9.37 

9.48 

8.91 

1.064 

" 

9.34 

9.46 

8.92 

1.061 

1  0'9 

" 

9.36 

9.44 

8.92 

1.058 

' 

" 

9.31 

9.39 

8.91 

1.054 

0.5 

11.66 

11.84 

11.14 

1.063 

] 

" 

11.73 

11.93 

" 

1.071 

L       1    O71 

" 

11.89 

12.02 

" 

1.079 

>       J.  .  Utf  1 

" 

11.79 

11.94 

" 

1.072 

1 

0.6 

14.12 
14.11 

14.37 
14.38 

13.37 

1.075 
1.076 

}     1.076 

0.7 

16.65 

16.91 

15.60 

1.084 

11    naft 

" 

16.71 

16.96 

" 

1.087 

1  .USD 

0.8 

19.16 
19.13 

19.50 
19.48 

17.82 
17.83 

1.094 
1.093 

}     1.094 

0.9 

21.92 
21.86 

22.34 
22.29 

20.06 
20.05 

1.114 
1.111 

}     1.113 

1.0 

24.53 

25.21 

22.28 

1.132 

] 

24.54 

25.37 

" 

1.139 

[     1.127 

24.27 

24.73 

22.29 

1.109 

The  osmotic  pressures  which  are  given  in  Table  18  are  those  which 
were  actually  observed,  that  is,  they  have  not  been  corrected  for 
inversion  or  dilution.  When  the  first  account  of  the  work  in  Series  III 
was  published,  it  was  still  imagined  that  inversion  might  be  responsible 
for  a  portion  of  the  loss  in  rotation,  though  it  was  conceded  that  a  con- 
siderable part  of  it  must  be  due  to  dilution.  Accordingly,  three  tentative 
tables  of  "corrected"  results  were  given.  In  one  of  them  the  whole  loss 
in  rotation  was  ascribed  to  inversion;  and  in  another,  to  dilution.  In  the 
third  table,  one-half  of  the  loss  was  ascribed  to  inversion  and  one-half  to 
dilution.  The  difference  between  the  corresponding  values  in  the  first 
and  second  tables  was  called  the  "limit  of  uncertainty"  as  to  the  true 


132       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

osmotic  pressure  of  the  solutions,  and  a  preference  was  expressed  for 
the  third  table,  in  which  a  compromise  had  been  attempted. 

When,  at  a  later  period,  it  was  proved  that  the  whole  loss  in  rotation 
had  been  due  to  dilution,  it  was  necessary  wholly  to  discard  the  first 
and  third  tables. 

Table  21  gives  the  results  of  Series  III  corrected  for  dilution  only.  It 
is  comparable  with  Table  17  for  Series  II.  The  corrected  pressures 
are  probably  somewhat  more  reliable  in  the  former  than  in  the  latter. 

SERIES  IV.* 

In  Series  IV  it  was  attempted  to  maintain  a  bath  temperature  of  5°, 
but  the  temperature  varied  as  a  rule  between  4°  and  5°.  On  two  occa- 
sions it  exceeded  6°  for  a  short  time.  Both  compartments  of  the  bath 
were  furnished  with  an  extensive  and  continuous  system  of  brass  pipes  for 
the  circulation  of  hydrant  water.  One-half  of  the  pipes  were  immersed 
in  the  water  in  the  lower  part  of  the  bath,  while  the  other  half  were  sus- 
pended from  the  top  of  the  upper,  or  manometer,  compartment.  The 
hydrant  water  entered  at  the  bottom,  and,  after  circulating  through  the 
whole  length  of  the  pipes  in  the  water,  it  ascended  and  traveled  through 
the  whole  length  of  the  system  in  the  air  space  before  escaping  from  the 
bath.  It  was  fed  to  the  bath  system  from  the  bottom  of  a  standpipe, 
4  meters  in  height,  and  its  rate  of  flow  was  regulated  by  means  of  a  valve 
placed  between  the  standpipe  and  the  bath.  In  order  that  the  pressure 
upon  the  water  circulating  in  the  bath  might  remain  constant,  also  that  it 
might  have,  at  the  time  of  entering,  the  temperature  of  the  water  in 
the  street  mains,  the  standpipe  was  provided  with  an  overflow  at  the 
top,  and  the  water  was  fed  into  it  as  directly  as  possible  from  the  main 
source  of  supply  for  the  building,  and  at  a  comparatively  high  rate.  The 
water  in  the  bath  in  which  the  lower  half  of  the  cooling  system  was 
submerged  was  kept  in  constant  circulation  by  means  of  a  pump. 

The  mean  mid-winter  temperature  of  the  water  in  the  street  mains 
is  about  4°,  and  no  difficulty  was  apprehended  in  maintaining  a  tem- 
perature of  5°  in  the  bath.  But  long  before  the  series  was  completed, 
the  temperature  of  the  hydrant  water  rose  above  5°,  and  it  was  neces- 
sary to  insert  a  system  of  pipes,  cooled  by  ice,  between  the  standpipe 
and  the  bath. 

The  cooling  system  described  above  is  essentially  the  same  as  that 
now  employed  in  all  baths  for  temperatures  above  0°  and  below  the 
highest  temperature  of  the  atmosphere,  only  it  has  been  found  better 
to  employ  two  independent  systems — one  for  the  lower  part  of  the  bath, 
where  the  cells  are  located,  and  another  for  the  air  or  manometer  space. 

During  the  work  upon  Series  IV,  the  cooling  system  was  in  the  experi- 
mental stage,  and  it  failed  to  operate  as  satisfactorily  as  it  afterwards, 
did  when  all  its  details  had  been  perfected.  This  accounts,  in  part,  for 

*Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer,  and  P.  B.  Dunbar.  Am.  Chem.  Jour., 
xxxvui,  175. 


CANE   SUGAR. 


133 


the  variations  in  bath  temperature,  which  ranged  between  0.2°  and  0.9°. 
The  principal  difficulty,  however,  was  due  to  the  fluctuating  external  tem- 
perature conditions,  which,  at  that  time,  were  not  under  good  control. 
The  observed  osmotic  pressures  are  given,  in  the  customary  form, 
in  Table  22. 

TABLE  22. — Cane  sugar,  Series  IV.    Extreme  bath  temperatures;  losses  in  rotation;  observed 
osmotic  pressures;  calculated  gas  pressures  of  the  solute. 


Concentration. 

Temperature. 

Loss  in 
rotation. 

Observed 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

degrees. 

degrees. 

0.1 

4.50  to  5.  30 
4.50       5.30 

0.10 
0.10 

2.40 
2.40 

2.27 

}     1.053 

0.2 

4.40       5.00 
5.75       6.45 

0.30 
0.05 

4.74 
4.76 

4.53 
4.56 

}     1.045 

0.3 

4.40       4.60 
4.40      4.60 

0.30 
0.30 

7.10 
7.04 

6.79 

}     1.041 

0.4 

4.20       5.10 
4.30       5.00 

0.40 
0.90 

9.44 
9.41 

9.05 

}     1.041 

0.5 

4.20       4.70 
5.00       5.50 

0.75 
0.75 

11.79 
11.85 

11.31 
11.35 

}     1.043 

0.6 

4.70       5.20 
5.75       6.45 

0.50 
0.70 

14.41 
14.45 

13.60 
13.66 

}     1.059 

0.7 

4.20       4.50 
4.40      4.75 

1.00 
1.40 

16.73 
16.85 

15.84 
15.85 

}     1.059 

0.8 

4.15       4.90 
4.25       4.40 

1.55 
1.40 

19.27 
19.34 

18.10 
18.09 

}     1.066 

0.9 

4.30       4.40 
5.00       5.50 

1.60 
1.65 

22.07 
22.22 

20.36 
20.42 

}     1.086 

1.0 

4.30       5.00 
4.20       4.55 

1.70 
2.10 

24.52 
24.53 

22.65 
22.62 

}     1.084 

Total 

=  17.  55  =4.  30  atmospheres.     Mean  =  1.058 

TABLE  23. — Cane  sugar,  Series  II  and  IV.    Fluctuations  in  bath  temperature. 


Concentration. 

Series  II. 

Series  IV. 

Concentration. 

Series  II. 

Series  IV. 

degree. 

degree. 

degree. 

degree. 

0.1 

0.10 

0.80 

0.6 

0.10 

0.50 

" 

0.10 

0.80 

" 

0.15 

0.70 

0.2 

0.95 

0.60 

" 

0.00 

" 

0.45 

0.70 

0.7 

0.65 

0.30 

" 

0.35 

" 

0.56 

0.35 

0.3 

0.45 

0.20 

0.8 

0.03 

0.75 

" 

0.15 

0.20 

" 

0.05 

0.15 

0.4 

0.30 

0.90 

0.9 

0.25 

0.10 

" 

0.10 

0.70 

" 

0.25 

0.50 

0.5 

0.20 

0.50 

1.0 

0.05 

0.70 

" 

0.00 

0.50 

" 

0.10 

0.35 

Means  .... 

0.24              0.52 

The  variations  in  bath  temperature  were  greater  in  Series  IV  than 
in  Series  III.  But  since  circulating  ice  water,  whose  temperature  is 
more  constant  than  that  of  hydrant  water,  was  used  in  the  latter,  it 
is  fairer  to  compare  Series  IV  with  Series  II,  if  with  any  other,  in  order 
to  ascertain  whether  any  substantial  progress  had  been  made  in  bath 


134 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


regulation.  This  is  done  in  Table  23,  from  which  it  appears  that  the 
mean  variation  in  bath  temperature  in  Series  IV  was  more  than  double 
that  in  Series  II,  as  if  the  means  of  bath  control  had  decreased,  instead 
of  increasing  in  efficiency.  It  is  easy  to  show,  however,  that  the  condi- 
tions to  be  met  in  the  case  of  Series  IV  were  more  difficult  than  in  that 
of  Series  II,  and  that,  on  this  account,  the  comparison  is  less  unfavorable 
to  the  former  than  it  appears  to  be.  Nevertheless,  it  was  considered 
necessary  to  revise  radically  the  system  of  bath  regulation  before 
beginning  the  next  series. 

If  Series  III  and  IV  are  compared  with  respect  to  the  upward  dis- 
placements of  the  manometers,  no  evidence  of  progress  is  to  be  detected. 
It  will  be  seen  in  Table  24  that  the  mean  displacements  were  about 
equal  in  the  two  series,  which  signifies  that  little  or  no  progress  had 
been  made  in  the  direction  of  fixing  the  capacity  of  the  cells. 

TABLE  24. — Cane  sugar,  Series  HI  and  IV.     Upward  displacements  of  the  manometers  (mm.}. 


Concentration. 

Series  III. 

Series  IV. 

Concentration. 

Series  III. 

Series  IV. 

mm. 

mm. 

mm. 

mm. 

0.1 

0.07 

0.40 

0.6 

0.48 

0.26 

" 

0.07 

0.34 

" 

0.10 

0.05 

" 

0.09 

0.7 

0.61 

0.23 

" 

0.09 

" 

0.45 

0.49 

0.2 

0.05 

0.22 

0.8 

0.01 

0.32 

" 

0.05 

0.04 

" 

0.34 

0.19 

0.3 

0.06 

0.07 

0.9 

0.44 

0.19 

" 

0.06 

0.07 

" 

0.05 

0.40 

0.4 

0.08 

0.03 

1.0 

0.23 

0.41 

" 

0.07 

0.15 

" 

0.59 

0.59 

" 

0.22 

" 

0.91 

u 

Of\A. 

0.5 

.  Urt 

0.24 

0.30 

Means.  .  .  . 

0.22 

0.24 

" 

0.22 

0.17 

" 

0.07 

" 

0.38 

If,  on  the  other  hand,  as  in  Table  25,  Series  III  and  IV  are  compared 
with  respect  to  loss  in  rotation,  which  is  the  measure  of  the  total 
dilution  which  the  solutions  suffered  while  in  the  cells,  some  improve- 
ment is  apparent.  The  relatively  smaller  loss  in  Series  IV  was  due 
to  improvements  in  manipulation  at  the  time  of  closing  and  opening 
the  cells,  particularly  during  the  latter  period. 

The  sum  of  the  rotations  of  all  the  20  solutions  used  in  Series  IV  was 
1249.00°.  The  loss  in  rotation  was  17.55°,  or  1 .41  per  cent.  The  loss  in 
rotation  in  Series  III  was  1.73  per  cent.  Expressed  in  terms  of  osmotic 
pressure,  the  dilution  in  Series  IV  was  equivalent  to  4.3  atmospheres, 
and  that  in  Series  III  to  6.74  atmospheres. 

Table  26  gives  the  results  of  Series  IV  as  corrected  for  dilution,  that 
is,  for  the  observed  losses  in  rotation.  It  stands  in  the  same  relation 
to  Series  IV  as  Table  17  to  Series  II,  and  Table  21  to  Series  III.  On 
the  whole,  the  corrected  osmotic  pressures  of  Series  IV  are  probably 
a  little  more  trustworthy  than  those  of  Series  III. 


CANE    SUGAR. 


135 


TABLE  25. — Cane  sugar,  Series  III  and  IV.    Losses  in  rotation. 


Concentration. 

Series  III. 

Series  IV. 

Concentration. 

Series  III. 

Series  IV. 

degrees. 

degrees. 

degrees. 

degrees. 

0.1 

0.20 

0.10 

0.6 

1.20 

0.50 

" 

0.10 

0.10 

" 

1.30 

0.70 

" 

0.05 

0.7 

1.20 

1.00 

" 

0.10 

" 

1.10 

1.40 

0.2 

0.15 

0.30 

0.8 

1.55 

1.55 

" 

0.15 

0.05 

" 

1.60 

1.40 

0.3 

0.50 

0.30 

0.9 

1.85 

1.60 

" 

0.60 

0.30 

" 

1.95 

1.65 

0.4 

0.55 

0.40 

1.0 

2.90 

1.70 

" 

0.60 

0.90 

" 

3.50 

2.10 

" 

0.40 
040 

" 

2.00 

0.5 

0.90 

0.75 

Totals  

27.65 

17  55 

i   nn 

071; 

.. 

1.05 

Per  cent  

1.78 

1.41 

M 

0.75 

Pressure  

6.75 

4  30 

TABLE  26. — Cane  sugar,  Series  IV.     Observed  osmotic  pressures  corrected  for  dilution,  and 
ratios  of  osmotic  to  calculated  gas  pressures  of  the  solute. 


Concen- 
tration. 

Observed 
osmotic 
pressure. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

Mean 
ratio. 

0.1 

2.40 
2.40 

2.42 
2.42 

2.27 

1.066 
1.066 

}     1.066 

0.2 

4.74 
4.76 

4.80 
4.77 

4.53 
4.56 

1.060 
1.046 

}     1.053 

0.3 

7.10 
7.04 

7.16 
7.10 

6.79 

1.055 
1.046 

}     1.051 

0.4 

9.44 
9.41 

9.50 
9.59 

9.05 

1.049 
1.060 

}     1.055 

0.5 

11.79 
11.85 

11.94 
12.00 

11.31 
11.35 

1.056 
1.057 

}     1.057 

0.6 

14.41 

14.51 

13.60 

1.067 

}1       f  W  W 

" 

14.45 

14.60 

13.66 

1.069 

1  .Uoo 

0.7 

16.73 
16.85 

16.94 
17.15 

15.84 
15.85 

1.070 
1.083 

}     1.077 

0.8 

19.27 
19.34 

19.61 
19.65 

18.10 
18.09 

1.084 
1.086 

}     1.085 

0.9 

22.07 
22.22 

22.44 
22.59 

20.36 
20.42 

1.102 
1.106 

}     1  .  104 

1.0 

24.52 
24.53 

24.91 
25.02 

22.65 
22.62 

1.100 
1.106 

}     1  .  103 

Mean 

=  1.072 

SERIES  V.* 

The  comparison  of  Series  II  and  IV  with  respect  to  temperature 
control  and  of  III  and  IV  with  respect  to  dilution  of  cell  contents  was, 
on  the  whole,  unsatisfactory.  There  was  to  be  found  in  the  results 
no  evidence  of  any  progress  in  the  improvement  of  devices  for  the 
regulation  of  the  baths,  and  the  reduction  of  dilution,  from  1.73  per 
cent  in  Series  III  to  1.41  per  cent  in  Series  IV,  did  not  betoken  an 
early  suppression  of  all  dilution.  It  was  evident  that,  in  order  to 

*Measurements  by  H.  N.  Morse  and  H.  V.  Morse.     Am.  Chem.  Jour.,  xxxiv,  667. 


136       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

accomplish  the  main  purposes  immediately  in  view — namely,  the  com- 
plete suppression  of  thermometer  effects  and  dilution — the  whole  method 
must  be  extensively  improved. 

The  revision  which  followed,  previous  to  beginning  Series  V,  was  a 
radical  one,  which  affected  nearly  every  detail  of  the  procedure.  The 
more  important  of  the  measures  taken  at  that  time  for  the  elimination 
of  dilution  have  already  been  mentioned.  The  method  of  wrapping 
the  exposed  part  of  the  stopper  was  changed,  with  the  result  that  in 
Series  V  and  in  the  succeeding  series  there  were  no  upward  displace- 
ments of  the  manometers.  In  other  words,  the  capacity  of  the  cells 
no  longer  increased  under  pressure,  and  one  of  the  three  sources  of 
dilution — though  probably,  in  the  beginning,  the  smallest — had  at  last 
been  eradicated.  The  practice  of  "dipping"  the  cells,  before  closing 
and  opening  them,  was  followed  systematically,  and  the  method  of 
piercing  and  "slitting"  the  stopper,  before  removing  the  manometer, 
was  greatly  improved.  It  was  at  this  time  also  that  nitrogen  was 
substituted  for  air  in  the  manometers,  and  that  more  attention  began 
to  be  given  to  the  errors  in  measurement  which  are  due  to  the  irregu- 
larities of  capillary  depression  in  narrow  tubes. 

The  improvements  in  the  devices  for  bath  regulation  had  in  view  the 
bringing  of  the  whole  system  of  temperature  control  into  harmony  with 
the  general  scheme  which  has  been  formulated  in  a  previous  chapter  in 
the  following  words: 

"If  all  the  water  or  air  in  a  bath  is  made  to  pass  rapidly  (1)  over  a  con- 
tinuously cooled  surface  which  is  capable  of  reducing  the  temperature  slightly 
below  that  which  it  is  desired  to  maintain,  then  (2)  over  a  heated  surface  which 
is  more  effective  than  the  cooled  one,  but  which  is  under  the  control  of  a 
thermostat,  and  (3)  again  over  the  cooled  surface,  etc.,  it  should  be  practicable 
to  maintain  in  the  bath  any  temperature  for  which  the  thermostat  is  set,  and 
the  constancy  of  the  temperature  should  depend  only  on  the  sensitiveness  of 
the  thermostat  and  the  rate  of  flow  of  the  water  or  air." 

The  essential  features  of  this  scheme — the  cooling  and  heating  sur- 
faces and  the  circulation  of  the  air  or  water  between  them — are  not 
novel.  They  are  exemplified  in  part  or  fully,  and  more  or  less  perfectly, 
in  nearly  all  baths.  But  perfect  success  in  temperature  regulation 
depends  upon  the  simultaneous  and  harmonious  cooperation  of  all 
three.  In  principle,  it  makes  no  difference  whether  the  heating  or 
cooling  agent  is  subjected  to  exact  regulation  by  a  thermostat. 

In  Series  I,  the  maintenance  of  temperature  was  by  insulation. 
There  was  no  thermostat  in  the  system — unless  the  insulation  can  be 
considered  in  that  light — and  the  walls  of  the  bath  became  therefore 
an  uncontrolled  heating  or  cooling  surface  according  to  the  temperature 
of  the  surrounding  air. 

In  Series  II,  the  heating  surface  was  provided  by  the  electric  stoves, 
which  were  regulated  by  a  thermostat.  The  other  essential — the  cooling 
surface — was  furnished  by  the  walls  of  the  bath;  but  these  became  an 


CANE   SUGAR.  137 

additional  but  uncontrolled  heating  surface  whenever  the  temperature  of 
the  air  rose  above  that  which  it  was  sought  to  maintain  in  the  bath. 

In  Series  III,  the  ice  water  was  the  cooling  agent  and  the  walls  of 
the  bath  were  the  heating  surface.  In  this  case  the  cooling  agent  was  regu- 
lated and  the  melting  ice  was  the  thermostat.  The  system  was  perfect 
in  principle,  but  failed  because  of  the  too  slow  circulation  of  the  water 
between  the  heating  and  refrigerating  surfaces. 

In  Series  IV,  the  hydrant  water  was  the  cooling  agent  and  the  bath 
walls  were  the  heating  surface.  As  in  Series  III,  the  cooling  agent, 
instead  of  the  heating  surface,  was  regulated.  In  Series  III,  the 
thermostat  was  melting  ice,  while  in  Series  IV,  it  was  the  valve  between 
the  stand-pipe  and  the  bath. 

Considered  as  a  thermostat  for  one  temperature  only,  nothing  is 
more  perfect,  of  course,  than  melting  ice,  except  a  liquid  of  constant 
boiling-point,  while  a  valve  regulating  the  flow  of  water  of  constant 
temperature  is  obviously  ineffective  unless  the  external  heat  supply 
is  constant  in  quantity.  The  system  of  cooling  employed  in  Series 
IV  was  excellent.  The  failure  to  regulate  satisfactorily  the  tempera- 
ture of  the  bath  was  due  to  the  fact  that  the  thermostat  (the  valve) 
was  not  sufficiently  automatic  in  its  action  to  overcome  the  inconstant 
external  temperature  conditions.  The  remedy  which  suggested  itself 
and  was  immediately  applied  was  the  reinstallation  in  the  bath  of  an 
electric  heating  system  controlled  by  a  mercury  thermostat.  The 
effect  of  this  was,  of  course,  to  give  the  regulation  of  the  bath  to  the 
heating  instead  of  the  cooling  system,  which  should  always  be  done 
unless  the  external  temperature  conditions  are  constant,  or  one  can 
employ  melting  ice  or  a  boiling  liquid. 

The  valve  did  not  become  useless  when  it  lost  its  character  as  a  thermo- 
stat, for  it  was  still  necessary  as  an  economizer  of  water  and  heat,  that 
is,  for  the  purpose  of  keeping  the  so-called  (t mar gin  of  under-cooling"  as 
small  as  practicable. 

The  beneficial  effect  of  the  improvement  in  manipulation  and  appa- 
ratus was  immediate  and  large.  In  Series  V,  the  loss  in  rotation  was 
small  and  was  confined  to  the  solutions  of  higher  concentration,  and 
the  fluctuations  in  bath  temperature  were  less  frequent  and  smaller 
than  in  any  previous  series. 

The  sum  of  the  rotations  of  all  the  solutions  in  Series  V  was  1249.6°. 
A  loss  of  2.50°  amounts  to  0.20  per  cent.  Expressed  in  osmotic  pressure, 
the  dilution  was  equivalent  to  about  0.64  atmosphere.  The  corre- 
sponding values  in  the  preceding  series  were  1249.00°,  17.55°,  1.41  per 
cent,  and  4.30  atmospheres.  The  sum  of  all  aberrations  in  bath  temper- 
ature was  1.40°  in  Series  V  and  10.30°  in  Series  IV.  There  were  no 
upward  displacements  of  the  manometers. 

In  Table  28  the  results  are  corrected  for  dilution  corresponding  to 
the  observed  losses  in  rotation. 


138 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


TABLE  27. — Cane  sugar,  Series  V.     Temperature  of  the  bath;  losses  in  rotation;  observed 
osmotic  pressures;  and  calculated  gas  pressures  of  the  solute. 


Concentration. 

Temperature. 

Loss  in 
rotation. 

Observed 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

degrees. 

degrees. 

0.1 

10.  00  to  10.  00 
10.00       10.00 

0.00 
0.00 

2.43 
2.44 

2.31 

}     1.054 

0.2 

10.00       10.20 
10.00       10.20 

0.00 
0.00 

4.81 
4.83 

4.62 

}     1.043 

0.3 

10.00       10.00 
10.00       10.10 

0.00 
0.00 

7.22 
7.15 

6.92 

}     1.038 

0.4 

10.00       10.10 
10.00       10.10 

0.00 
0.00 

9.53 
9.61 

9.24 

}     1.036 

0.5 

10.00       10.00 
10.00       10.00 

0.00 
0.10 

11.96 
12.02 

11.54 

}     1.039 

0.6 

10.10       10.10 
10.10       10.10 

0.00 
0.00 

14.53 
14.55 

13.85 

}     1.051 

0.7 

10.30       10.10 
10.00       10.10 

0.10 
0.00 

17.10 
17.08 

16.16 

}     1.058 

0.8 

9.90       10.00 
10.00       10.00 

0.20 
0.10 

19.73 
19.77 

18.47 

}     1.069 

0.9 

10.00       10.00 
10.00       10.00 

0.40 
0.40 

22.28 
22.28 

20.77 

}     1.073 

1.0 

10.  €0       10.30 
10.00       10.20 

0.60 
0.60 

25.08 
25.03 

23.10 

}     1.085 

Total 

=  2.50 

Mean 

=  1.055 

TABLE  28. — Cane  sugar,  Series  V.    Observed  osmotic  pressures  corrected  for  dilution,  and 
ratios  of  osmotic  to  calculated  gas  pressures  of  the  solute. 


Concen- 
tration. 

Observed 
osmotic 
pressure. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

Mean 
ratio. 

0.1 

2.43 

2.44 

2.43 

2.44 

2.31 

1.052 
1.056 

}     1.054 

0.2 

4.81 
4.83 

4.81 
4.83 

4.62 

1.041 
1.045 

}     1.043 

0.3 

7.22 
7.15 

7.22 
7.15 

6.92 

1.043 
1.033 

}     1.038 

0.4 

9.53 
9.61 

9.53 
9.61 

9.24 

1.031 
1.040 

}     1.036 

0.5 

11.96 
12.02 

11.96 
12.04 

11.54 

1.036 
1.043 

}     1.040 

0.6 

14.53 
14.55 

14.53 
14.55 

13.85 

1.050 
1.051 

}     1.051 

0.7 

17.10 
17.08 

17.13 

17.08 

16.16 

1.060 
1.057 

}     1.058 

0.8 

19.73 
19.77 

19.77 
19.80 

18.47 

1.070 
1.072 

}     1.071 

0.9 

22.28 

22.37 

20.77 

1.077 

\     i  077 

" 

22.28 

22.37 

" 

1.077 

1.0 

25.08 
25.03 

25.23 
25.18 

23.10 

1.092 
1.090 

}     1.091 

Mean 

=  1.056 

CANE    SUGAR. 


139 


SERIES  VI.* 

The  conditions  under  which  the  measurements  of  Series  VI  were  made 
were  essentially  the  same  as  in  Series  V.  Some  improvements  had  been 
made  in  the  interval  between  the  two  in  both  the  cooling  and  heating 
systems,  and  the  circulation  of  the  bath  water  surrounding  the  lower 
half  of  the  cooling  system  had  been  made  more  effective.  Some  slight 
improvements  had  also  been  made  in  the  manipulation.  The  beneficial 
effect  of  the  alterations  is  shown  in  the  smaller  fluctuations  of  bath  tem- 
perature and  in  the  diminished  loss  in  rotation.  Except  in  one  case, 
the  dilution  was  confined  to  the  solutions  of  higher  concentration. 

TABLE  29. — Cane  sugar,  Series  VI.     Bath  temperatures;  losses  in  rotation;  observed  osmotic 
pressures;  and  calculated  gas  pressures  of  the  solute. 


Concentration. 

Temperature. 

Loss  in 
rotation. 

Observed 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

0.1 

degrees. 
15.  00  to  15.00 
15.00       15.00 

degrees. 
0.00 
0.00 

2.47 
2.48 

2.35 

}     1.054 

0.2 

15.00       15.00 
15.00       15.00 

0.10 
0.00 

4.92 
4.90 

4.70 

}     1.045 

0.3 

15.00       15.00 
15.00       15.00 

0.00 
0.00 

7.31 
7.35 

7.05 

}     1.040 

0.4 

15.00       15.10 
15.00       15.10 

0.00 
0.00 

9.77 
9.78 

9.40 

}     1.040 

0.5 

15.00       15.00 
15.00       15.10 

0.00 
0.00 

12.29 
12.29 

11.75 

}     1.046 

0.6 

15.00       15.00 
15.00       15.00 

0.00 
0.00 

14.91 
14.81 

14.09 

}     1.055 

0.7 

15.00       15.00 
15.00       15.10 

0.10 
0.05 

17.42 
17.36 

16.44 

}     1.058 

0.8 

15.00       15.00 
15.00       15.00 

0.20 
0.15 

20.07 
20.11 

18.79 

}     1.069 

0.9 

15.00       15.00 
15.00       15.00 

0.30 
0.15 

22.97 
22.91 

21.14 

}     1.085 

1.0 

15.00       15.00 
15.00       15.00 

0.00 
0.30 

25.39 
25.44 

23.49 

}     1.082 

Sum 

=  1.35 

Mean 

=  1.057 

Table  29  gives  the  temperatures,  the  losses  in  rotation,  the  observed 
osmotic  pressures  of  the  solutions,  and  the  calculated  gas  pressures  of 
the  solute. 

In  Series  VI,  the  sum  of  all  the  original  rotations  was  1249.59°.  A  loss 
of  1.35°  amounts  to  0.11  per  cent,  or  a  dilution  equivalent  to  0.34  atmos- 
phere for  the  whole  series.  The  corresponding  values  for  Series  V  were 
1249.60°,  2.50°,  0.20  per  cent,  and  0.64  atmosphere.  The  sum  of  all 
the  fluctuations  in  bath  temperature  was  0.4°  in  Series  VI,  and  1.4°  in 
Series  V.  There  were  no  upward  displacements  of  the  manometer. 

In  Table  30  the  observed  pressures  are  corrected  for  the  small  losses 
in  rotation.  

*Measurements  by  H.  N.  Morse  and  B.  Mears.     Am.  Chem.  Jour.,  XL,  194. 


140 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


TABLE  30. — Cane  sugar,  Series  VI.    Observed  osmotic  pressures  corrected  for  dilution,  and 
ratios  of  osmotic  to  calculated  gas  pressure  of  the  solute. 


Concen- 
tration. 

Observed 
osmotic 
pressure. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

Mean 
ratio. 

0.1 

2.47 

2.47 

2.35 

1.052 

}i    ni/i 

" 

2.48 

2.48 

" 

1.055 

1  .\Jfrk 

0.2 

4.92 

4.94 

4.70 

1.051 

}i    r\Af 

" 

4.90 

4.90 

" 

1.043 

i  .\y±i 

0.3 

7.31 
7.35 

7.31 
7.35 

7.05 

1.037 
1.043 

}     1.040 

0.4 

9.77 
9.78 

9.77 
9.78 

9.40 

1.039 
1.040 

}     1.040 

0.5 

12.29 

12.29 

11.75 

1.046 

>i    n  i  A 

" 

12.29 

12.29 

" 

1.046 

1  .  U4O 

0.6 

14.91 

14.91 

14.09 

1.058 

\     i  055 

" 

14.81 

14.81 

" 

1.051 

0.7 

17.42 
17.36 

17.44 
17.37 

16.44 

1.061 
1.057 

}     1.059 

0.8 

20.07 

20.11 

18.79 

1.070 

I     1.071 

" 

20.11 

20.14 

" 

1.072 

0.9 

22.97 
22.91 

23.04 
22.98 

21.14 

1.090 
1.087 

}     1.089 

1.0 

25.39 
25.44 

25.39 
25.51 

23.49 

1.081 
1.086 

}     1.083 

Mean  =  1.058 

SERIES  VII.* 

While  the  work  in  Series  V  and  VI  was  in  progress,  certain  defects 
of  construction  in  the  cooling  system  manifested  themselves.  The 
principal  difficulty  experienced  in  this  connection  was  with  the  gas 
which  was  expelled  from  the  hydrant  water  while  passing  through  the 
cooling  system.  Provision  had  been  made  in  the  beginning  for  the 
easy  escape  of  this  gas  by  giving  all  the  pipes  in  the  cooling  system  a 
slight  upward  inclination  in  the  direction  in  which  the  water  was  to 
run.  It  was  found,  however,  that,  despite  the  inclined  position  of  the 
pipes  and  the  arrangement  for  constant  pressure,  no  perfectly  steady 
flow  of  water  could  be  secured  when  the  stream  passing  through  the 
system  was  small.  When  it  was  very  small,  the  flow  of  water  would 
cease  altogether  in  a  few  hours.  It  was  necessary,  therefore,  at  stated 
intervals  throughout  the  day  and  night,  to  open  wide  for  a  few  seconds 
the  regulating  valve  and  flush  the  system.  The  cause  of  the  difficulty 
was  finally  located  in  the  valve  itself,  which  was  found  to  be  of  such 
construction  that  gas  could  accumulate  in  it  and  eventually  stop  the 
flow  of  water  whenever  the  stream  passing  through  the  system  was 
small.  The  valve  was  replaced  by  one  of  different  construction,  and 
no  further  difficulty  was  experienced  in  securing  a  constant  flow  of  water. 

There  was  no  material  variation  in  the  temperature  of  the  bath 
during  any  experiment  in  Series  VII,  and  it  was  apparent,  therefore, 
that  one  of  the  two  great  objects  of  the  preliminary  investigation  had 
been  accomplished.  The  system  of  bath  regulation  had  been  improved 
to  the  point  where  thermometer  effects  were  no  longer  to  be  feared.  Dilu- 
tion, on  the  other  hand,  had  not  been  entirely  suppressed.  In  a  total 

*Measurements  by  H.  N.  Morse  and  W.  W.  Holland.    Am.  Chem.  Jour.,  xu,  1. 


CANE   SUGAR. 


141 


rotation  of  1246.85°,  the  loss  was  1.30°,  or  0.10  per  cent,  which  was 
equivalent  in  terms  of  osmotic  pressure  to  a  dilution  of  0.31  atmosphere. 
In  Series  VI,  the  dilution  amounted  to  0.1 1  per  cent,  or  0.34  atmosphere. 
The  observed  pressures  of  Series  VII  are  given  in  Table  31 : 

TABLE  31. — Cane  sugar,  Series  VII.     Temperature;  observed  osmotic  pressures; 
losses  in  rotation;  and  calculated  gas  pressures  of  the  solute. 


Concen- 
tration. 

Tempera- 
ture. 

Loss  in 
rotation. 

Observed 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

degrees. 

degrees. 

0.1 

25.00 

0.00 

2.56 
2.56 

2.43 

}     1.053 

0.2 

,. 

.. 

5.09 
5.10 

4.86 

}     1.048 

0.3 

.. 

.. 

7.58 
7.55 

7.29 

}     1.038 

0.4 

it 

.. 

10.10 
10.13 

9.72 

}     1.041 

0.5 

.. 

.. 

12.75 
12.71 

12.15 

[     1.048 

0.6 

ii 

.. 

15.43 
15.41 

14.58 

[     1.058 

0.7 

.. 

0.10 
0.10 

18.03 
18.02 

17.02 

[     1.059 

0.8 

.. 

0.00 
0.20 

20.75 
20.71 

19.45 

[     1.066 

0.9 

.. 

0.15 
0.25 

23.71 
23.67 

21.88 

[     1.082 

1.0 

" 

0.10 
0.40 

26.33 
26.39 

24.31 

>     1.084 

Sum 

=  1.30 

Mean 

=  1.058 

Table  32  gives  the  observed  pressures  as  corrected  for  dilution: 

TABLE  32. — Cane  sugar,  Series  VII.    Observed  osmotic  pressures  corrected  for 
dilution,  and  ratios  of  osmotic  to  calculated  gas  pressures  of  the  solute. 


Concen- 
tration. 

Observed 
osmotic 
pressure. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

Mean 
ratio. 

0.1 

2.56 
2.56 

2.56 
2.56 

2.43 

1.053 
1.053 

}     1.053 

0.2 

5.09 
5.10 

5.09 
5.10 

4.86 

1.047 
1.049 

}     1.048 

0.3 

7.58 
7.55 

7.58 
7.55 

7.29 

1.040 
1.036 

}     1.038 

0.4 

10.10 
10.13 

10.10 
10.13 

9.72 

1.039 
1.042 

}     1.041 

0.5 

12.75 

12.75 

12.15 

1.049 

i      .   ».,, 

" 

12.71 

12.71 

" 

1.046 

/     1>U48 

0.6 

15.43 

15.43 

14.58 

1.058 

1  .058 

" 

15.41 

15.41 

" 

1.057 

0.7 

18.03 
18.02 

18.05 
18.04 

17.02 

1.061 
1.060 

}     1.060 

0.8 

20.75 
20.71 

20.75 
20.76 

19.45 

1.067 
1.067 

}     1.067 

0.9 

23.71 
23.71 

23.75 
23.75 

21.88 

1.085 
1.085 

}     1.085 

1.0 

26.33 

26.35 

24.31 

1.084 

I       i     f\G7 

" 

26.39 

26.49 

" 

1.090 

1  .  Uo« 

Mean  =  1.059 

142 


OSMOTIC   PRESSURE   OF   AQUEOUS   SOLUTIONS. 


SERIES  VIIL* 

The  loss  in  rotation  in  Series  VI  amounted  to  0.11  per  cent  and  to  0.10 
per  cent  in  Series  VII.  There  was  little  prospect  of  further  improve- 
ment in  the  manipulation  concerned  in  the  closing  and  opening  of  the 
cells,  and  it  was  therefore  concluded  that  the  continued  small  dilution 
of  the  more  concentrated  solutions  could  not  be  wholly  suppressed  as 
long  as  the  rubber  stopper  was  retained  as  one  of  the  features  of  the 
cell.  Various  devices  for  closing  the  cell  without  it  had  been  studied 
and  more  or  less  tested,  but  none  of  them  had  proved  to  be  entirely 
practicable  except  the  forerunner  of  the  arrangement  seen  in  Figure  9, 

TABLE  33. — Cane  sugar,  Series  VIII .     Temperature;  observed  osmotic  pressures;  calculated 
gas  pressures  of  solute;  and  ratios  of  osmotic  to  gas  pressures. 


Concen- 
tration. 

Tempera- 
ture. 

Observed 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

Mean 
ratio. 

degrees. 

0.1 

20.00 

2.53 
2.52 

2.39 

1.059 
1.054 

}     1.057 

0.2 

•' 

5.03 

4.78 

1.052 

i 

" 

•• 

5.02 

" 

1.050 

\     1.050 

11 

i 

5.02 

" 

1.050 

J 

0.3 

, 

7.45 
7.45 

7.17 

1.039 
1.039 

}     1.039 

0.4 

1 

9.98 

9.56 

1.044 

1 

" 

' 

9.94 

" 

1.040 

\     1.042 

" 

• 

9.97 

" 

1.043 

I 

0.5 

• 

12.49 

11.95 

1.045 

1 

" 

1 

12.49 

" 

1.045 

[     1.045 

" 

' 

12.50 

" 

1.046 

j 

0.6 

, 

15.18 
15.22 

14.34 

1.059 
1.061 

}     1.060 

0.7 

• 

17.83 

16.73 

1.066 

1 

" 

• 

17.85 

" 

1.067 

\     1.066 

" 

1 

17.83 

" 

1.066 

] 

0.8 

1 

20.57 

19.12 

1.076 

} 

" 

" 

20.62 

" 

1.078 

\     1.077 

" 

" 

20.62 

" 

1.076 

I 

0.9 

" 

23.36 

21.51 

1.086 

\ 

" 

" 

23.31 

" 

1.084 

\     1.084 

" 

" 

23.27 

" 

1.082 

J 

1.0 

" 

26.12 

23.90 

1.093 

] 

" 

" 

26.13 

" 

1.093 

\     1.093 

" 

" 

26.11 

" 

1.093 

J 

Mean 

=  1.061 

page  19,  which  was  altered  and  unproved  until  it  became  satisfactory. 
The  requirements  of  such  a  device  were  that  it  should  permit  of  a 
practically  instantaneous  closing  or  opening  of  the  cells,  and  that  it 
should  render  an  enlargement  of  cell  capacity  under  pressure  impossible. 
In  general,  it  is  difficult  to  meet  the  second  requirement  if  any  rubber 
whatever  is  used  in  closing  the  cells,  and  we  should  have  been  glad  to 
dispense  with  that  material  altogether.  But  after  securely  closing  a 
cell,  it  is  necessary  to  bring  upon  the  contents  an  initial  pressure  which 
is  nearly  equal  to,  or  even  a  little  above,  the  osmotic  pressure  of  the 

*Measurements  by  H.  N.  Morse  and  W.  W.  Holland.     Am.  Chem.  Jour.,  XLI,  257. 


CANE   SUGAR. 


143 


solution.  Otherwise,  much  time  is  lost  in  waiting  for  equilibrium,  and 
some  dilution  occurs  in  consequence  of  the  compression  of  the  gas  in 
the  manometer.  Up  to  the  present  time,  however,  the  writer  has  been 
unable  to  devise  a  successful  means  for  producing  this  initial  pressure 
which  did  not  involve  the  use  of  rubber. 

Throughout  Series  VII  and  VIII,  the  temperatures  of  the  bath  were 
constant,  and  with  the  introduction  into  the  latter  of  the  new  device 
for  closing  the  cells,  the  last  traces  of  loss  in  rotation  disappeared. 
With  Series  VIII,  therefore,  the  four  years'  struggle  against  thermometer 
effects  and  dilution  was  brought  to  a  successful  issue. 

The  progress  of  the  work  from  the  beginning  to  the  end  of  the 
endeavor  to  eliminate  the  large  sources  of  error  from  the  direct  method 
of  measuring  osmotic  pressure  is  summarized,  and  can  be  reviewed  at  a 
glance  in  Tables  34,  35,  and  36. 

TABLE  34. — Cane  sugar,  Series  I  to  VIII.     Fluctuations  in  bath  temperature. 


Concen- 
tration. 

Series 
I. 

Series 
II. 

Series 
III. 

Series 
IV. 

Series 
V. 

Series 
VI. 

Series 
VII. 

Series 
VIII. 

0.1 

degrees. 
4.48 
1.36 

degrees. 
0.05 
0.05 

degrees. 
0.18 
0.12 
0  24 

degrees. 
0.80 
0.80 

degrees. 
0.00 
0.00 

degrees. 
0.00 
0.00 

degrees. 
0.00 
0.00 

degrees. 
0.00 
0.00 

it 

0  24 

0.2 

2.11 
0.42 

0.50 
0.53 
0  35 

0.10 
0.04 

0.60 
0.65 

6.20 
0.20 

0.30 
0.10 

0.00 
0.00 

0.00 
0.00 
0.00 

0.3 
0.4 

1.44 
1.60 
0.40 
0.40 

0.15 
0.40 
0.27 
0.10 

0.16 
0.04 
0.06 
0.10 
0.08 

0.15 
0.15 
0.90 
0.40 

0.00 
0.10 
0.10 
0.10 

0.10 
0.00 
0.10 
0.10 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 
0.00 

ii 

0  04 

0.5 

0.16 
0.88 

0.20 
0.00 

0.24 
0.16 
0.06 

0.50 
0.50 

0.10 
0.00 

0.00 
0.10 

0.00 
0.00 

0.00 
0.00 
0.00 

ii 

0.04 

0.6 

0.70 
1.10 

0.10 
0.15 
0  00 

0.12 
0.05 
0  00 

0.50 
0.70 

0.00 
0.00 

0.00 
0.00 

6.66 

0.00 

0.00 
0.00 

0.7 

0.90 
1.90 

0.65 
0.20 

0.12 
0.05 

0.30 
0.35 

0.10 
0.10 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

0.8 
ii 

0.92 
1.00 

0.03 
0.05 

0.10 
0.10 

0.75 
0.15 

0.20 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

0.9 

1.10 

0.25 
0.25 

0.03 
0.10 

o.io 

0.50 

0.00 
0.00 

6.66 

0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

1.0 

1.80 
1.10 
1  10 

0.07 

0.14 
0.10 

0.70 
0.35 

0.30 
0.20 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

Totala.  .  . 
Means.  . 

29.14 
1.22 

4.35 
0.21 

2.81 
0.10 

9.85 
0.49 

1.70 
0.08 

0.80 
0.04 

0.00 
0.00 

0.00 
0.00 

Table  34  gives  the  fluctuations  hi  bath  temperature  for  each  of  the 
175  experiments  of  Series  I  to  VIII,  inclusive.  The  test  and  the 
measure  of  progress  in  the  improvement  of  the  facilities  for  the  main- 


144 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


tenance  of  temperature  are,  in  a  general  way,  the  diminution,  from 
series  to  series,  in  the  fluctuations  of  bath  temperature.  It  has  already 
been  pointed  out,  however,  that  certain  series  can  not  be  fairly  judged 
by  such  a  comparison. 

Variations  in  bath  temperature  during  the  individual  experiments 
are  only  a  rough  measure  of  thermometer  effects.  The  very  complex 
character  of  these  phenomena  and  their  highly  pernicious  effects  upon 
the  precision  of  the  measurements  of  osmotic  pressure  have  been  dis- 
cussed in  a  former  chapter,  and  it  is  not  necessary  again  to  emphasize 
the  necessity  of  their  eliminiation. 

When,  as  in  Series  VII  and  VIII,  no  fluctuations  are  given,  it  is  not 
meant  thereby  that  the  temperature  was  absolutely  constant  through- 
out, but  simply  that  the  variation  was  less  than  0.05°.  In  the  "final 
measurements"  to  be  presented  in  later  chapters,  it  will  mean  that  the 
variation  was  less  than  0.02°. 

TABLE  35. — Cane  sugar,  Series  I  to  VIII,     Upward  displacements  of  the  manometers  (mm.}. 


Concen- 
tration. 

Series 
I. 

Series 
II. 

Series 
III. 

Series 
IV. 

Series 
V. 

Series 
VI. 

Series 
VII. 

Series 
VIII. 

0.1 

(*) 
(*) 
(*) 

0.07 
0.54 

0.07 
0.07 
0.09 

0.40 
0.34 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

•• 

(*) 

0.09 

0.2 

(*) 
(*) 
(*) 

2.78 
1.44 
0.19 

0.05 
0.05 

0.22 
0.04 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0  00 

0.3 
0.4 

(*) 
(*) 
(*) 
(*) 
(*) 

0.12 
0.10 
0.30 
1.18 

0.06 
0.06 
0.08 
0.07 
0.22 

0.07 
0.07 
0.03 
0.15 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 
0.00 

<> 

(*) 

0.04 

0.5 

(*) 
(*) 
(*) 

0.16 
0.32 

0.24 
0.22 
0.07 

0.30 
0.17 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

«• 

(*) 

0.38 

0.6 

(*) 
(*) 
(*) 

3.78 
0.83 
0.64 

0.48 
0.10 

0.26 
0.05 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.7 

(*) 
(*) 
(*) 

0.24 
0.07 

0.61 
0.45 

0.23 
0.49 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

0.8 

(*) 
(*) 
(*) 

0.32 
1.80 

0.01 
0.34 

0.32 
0.19 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

0.9 

(*) 
(*) 
(*) 

1.21 

0.44 
0.05 

0.19 
0.40 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

1.0 

(*) 
(*) 

(*) 

0.88 
2.97 

0.23 
0.59 
0.91 

0.41 
0.59 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

Means.  . 

0.94 

0.22 

0.24 

*Not  determined. 


CANE   SUGAR. 


145 


Tables  35  and  36  summarize  the  progress  made  in  suppressing  dilu- 
tion. The  first  gives  the  upward  displacements  of  the  manometers 
which  attended  distortions  of  the  rubber  stoppers  under  pressure. 
They  are  to  be  regarded  merely  as  a  symptom  of  such  distortion,  and 
not  as  a  measure  of  the  increase  in  the  capacity  of  the  cells.  The  more 
important  of  the  two  tables  is  36,  which  gives  the  losses  in  rotation, 
that  is,  the  amounts  of  dilution  from  all  sources  which  the  solutions 
suffered  while  in  the  cells. 

TABLE  36. — Cane  sugar,  Series  I  to  VIII.    Loss  in  rotation  (degrees). 


Concen- 
tration. 

Series 
I. 

Series 
II. 

Series 
III. 

Series 
IV. 

Series 
V. 

Series 
VI. 

Series 
VII. 

Series 
VIII. 

0.1 

0.2 

0.3 
0.4 

0.5 

0.6 
0.7 
0.8 
0.9 
1.0 

Totals  . 

(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 

0.50 
0.50 

o.7o 

0.70 
0.50 
0.50 
0.60 
0.80 
1.30 

1.40 
1.20 

2^80 
1.90 
1.80 
2.60 
2.20 

0.20 
0.10 
0.05 
0.10 
0.15 
0.15 

0.10 
0.10 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.30 
0.05 

0.00 
0.00 

0.10 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 

0.50 
0.60 
0.55 
0.60 
0.40 
0.40 
0.90 
1.00 
1.05 
0.75 
1.20 
1.30 

0.30 
0.30 
0.40 
0.90 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 

0.75 
0.75 

0.00 
0.10 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 
0.00 

0.50 
0.70 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

0.00 
0.00 

1.20 
1.10 

1.00 
1.40 

0.10 
0.00 

0.10 
0.05 

0.10 
0.10 

0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 
0.00 

3.20 
3.90 

1.55 
1.60 

1.55 
1.40 

0.20 
0.10 

0.20 
0.15 

0.00 
0.20 

2.50 
2.40 

1.85 
1.95 

1.60 
1.65 

0.40 
0.40 

0.30 
0.15 

0.15 
0.25 

2.40 
4.00 

2.90 
3.50 
2.00 

1.70 
2.10 

0.60 
0.60 

0.00 
0.30 

0.10 
0.40 

38.40 
2.86 
9.37 

27.65 
1.73 
6.74 

17.55 
1.41 
4.30 

2.50 
0.20 
0.64 

1.35 
0.11 
0.34 

1.30 
0.10 
0.31 

0.00 
0.00 
0.00 

Per  cent 
Osmotic 

pressure. 

*Loss  in  rotation  not  determined. 


The  main  object  in  view  during  the  second  period  of  the  investigation 
was  the  development  of  the  method — specifically,  the  suppression  of 
thermometer  effects  and  dilution — and  hitherto  the  data  of  Series  I  to 
VIII  have  been  arranged  or  discussed  principally  with  reference  to  the 
progress  made  in  that  direction.  The  present  chapter  might  properly 


146 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


be  concluded  at  this  point.  But,  since  the  results  of  the  measurements 
foreshadow  much  that  was  afterwards  established  by  means  of  greater 
precision,  it  has  been  thought  worth  while  to  arrange  them  in  Tables 
37  to  39,  with  a  view  to  ascertaining  what  general  conclusions  they 
suggest  with  reference  to  the  osmotic  pressure  of  cane-sugar  solutions. 
Section  A  of  Table  37  gives  all  of  the  observed  osmotic  pressures 
of  Series  I  to  VIII,  except  those  of  the  0.05  and  0.25  concentrations  of 
Series  I;  these  concentrations  are  omitted,  as  they  were  abandoned 
after  the  first  series.  Section  B  gives  the  observed  osmotic  pressures 
of  Section  A,  corrected  for  dilution  proportional  to  the  observed  losses 
in  rotation.  Section  C  gives,  for  each  experiment,  the  ratio  of  the  cor- 
rected osmotic  pressure  to  the  calculated  gas  pressure  of  the  solute. 

TABLE  37. — Cane  sugar,  Series  I  to  VIII.     SECTION  A.  OBSERVED  OSMOTIC  PBESSURES. 


Cone. 

Series  I. 
17°-25°. 

Series  II. 
20°-24°. 

Series  III. 
0.12°-0.38°. 

Series  IV. 
4°-5°. 

Series  V. 
10°. 

Series  VI. 
15°. 

Series  VII. 
25°. 

Series  VIII. 
20°. 

0.1 

2.37 
2.44 

2.58 
2.62 

2.45 
2.45 
2.37 

2.40 
2.40 

2.43 
2.44 

2.47 

2.48 

2.56 
2.56 

2.53 
2.52 

H 

2.39 

0.2 

4.77 
4.83 

4.75 

4.82 
4.88 

4.78 
4.77 

4.74 
4.76 

4.81 
4.83 

4.92 
4.90 

5.09 
5.10 

6.03 
5.02 
5.02 

0.3 
0.4 

7.23 
7.23 
9.51 
9.72 

7.28 
7.31 
9.76 
9.71 

7.09 
7.11 
9.37 
9.34 
9.36 

7.10 
7.04 
9.44 
9.41 

7.22 
7.15 
9.53 
9.61 

7.31 
7.35 
9.77 

9.78 

7.58 
7.55 
10.10 
10.13 

7.45 
7.45 
9.98 
9.94 
9.97 

H 

9.31 

0.5 

12.02 
12.17 

12.28 
12.41 

11.66 
11.73 
11.89 

11.79 
11.85 

11.96 
12.02 

12.29 
12.29 

12.75 
12.71 

12.49 
12.49 
12.50 

H 

11.79 

0.6 

14.34 

14.57 

14.82 
15.00 
15.06 

14.12 
14.11 

14.41 
14.45 

14.53 
14.55 

14.91 
14.81 

15.43 
15.41 

15.18 
15.22 

0.7 

16.79 
17.02 

17.38 
17.32 

16.55 
16.71 

16.73 
16.85 

17.10 
17.08 

17.42 
17.36 

18.03 
18.02 

17.83 
17.85 
17.83 

0.8 

19.39 
19.54 

19.83 
19.77 

19.16 
19.13 

19.27 
19.34 

19.73 
19.77 

20.07 
20.11 

20.75 
20.71 

20.57 
20.62 
20.62 

0.9 

21.89 

22.35 
22.32 

21.92 
21.86 

22.07 
22.22 

22.28 
22.28 

22.97 
22.91 

23.71 
23.67 

23.36 
23.31 
23.27 

1.0 

24.80 
24.39 
24.50 

24.83 

24.78 

24.53 

24.54 
24.57 

24.52 
24.53 

25.08 
25.03 

25.39 
25.44 

26.33 
26.39 

26.12 
26.13 
26.11 

CANE   SUGAR. 


147 


TABLE  37. — SECTION  B.  OBSERVED  OSMOTIC  PRESSURES  CORRECTED  FOR  DILUTION. 


Cone. 

Series  I. 
17°-25°. 

Series  II. 
20°-24°. 

Series  III. 
0.12°-0.38°. 

Series  IV. 
4°-5°. 

Series  V. 
10°. 

Series  VI. 
15°. 

Series  VII. 
25°. 

Series  VIII. 
20° 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 
0.7 
0.8 
0.9 
1.0 

(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 

2.69 
2.73 

2.49 
2.47 
2.38 
2.41 

4.81 
4.80 

2.42 
2.42 

2.43 

2.44 

2.47 
2.48 

2.56 
2.56 

2.53 
2.52 

4.89 
4.96 
4.98 
7.38 
7.43 
9.92 
9.98 

4.80 
4.77 

4.81 
4.83 

4.94 
4.90 

5.09 
5.10 

5.03 
5.02 
5.02 
7.45 
7.45 
9.98 
9.94 
9.97 

7.19 
7.13 
9.48 
9.46 
9.44 
9.39 
11.84 
11.93 
12.02 
11.94 
14.37 
14.38 

7.16 
7.10 
9.50 
9.59 

7.22 
7.15 
9.53 
9.61 

7.31 
7.35 
9.77 
9.78 

7.58 
7.55 
10.10 
10.13 

12.58 
12.67 

11.94 
12.00 

11.96 
12.04 

12.29 
12.29 

12.75 
12.71 

12.49 
12.49 
12.50 

15.44 
15.42 
15.46 
17.96 
17.81 

14.51 
14.60 

14.53 
14.55 

14.91 
14.81 

15.43 
15.41 

15.18 
15.22 

16.91 
16.96 

16.94 
17.15 

17.13 
17.08 

17.44 
17.37 

18.05 
18.04 

17.83 
17.85 
17.83 
20.57 
20.62 
20.62 
23.36 
23.31 
23.27 
26.12 
26.13 
26.11 

20.57 
20.68 

19.50 
19.48 

19.61 
19.65 

19.77 
19.80 

20.11 
20.14 

20.75 
20.76 

22.83 
22.67 

22.34 
22.29 

22.44 
22.59 

22.37 
22.37 

23.04 
22.98 

23.75 
23.75 

25.43 
25.74 

25.21 
25.37 
24.73 

24.91 
25.02 

25.23 
25.18 

25.39 
25.51 

26.35 
26.49 

SECTION  C.  RATIOS  OF  CORRECTED  OSMOTIC  TO  GAS  PRESSURE  OF  SOLUTE. 

0.1 

0.2 

0.3 
0.4 

0.5 

0.6 

0.7 
0.8 
0.9 
1.0 

(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
(*) 
<*) 
(*) 

1.114 
1.124 

1.117 
1.108 
1.067 
1.081 
1.078 
1.076 

1.066 
1.066 

1.052 
1.056 

1.052 
1.055 

1.053 
1.053 

1.059 
1.054 

1.021 
1.033 
1.021 
1.031 
1.031 
1.032 
1.036 

1.060 
1.046 

1.041 
1.045 

1.051 
1.043 

1.047 
1.049 

1.052 
1.050 
1.050 
1.039 
1.039 
1.044 
1.040 
1.043 

1.076 
1.067 
1.064 
1.061 
1.058 
1.054 
1.063 
1.071 
1.079 
1.072 
1.075 
1.076 

1.055 
1.046 
1.049 
1.060 

1.043 
1.033 
1.031 
1.040 

1.037 
1.043 
1.039 
1.040 

1.040 
1.036 
1.039 
1.042 

1.043 
1.047 

1.056 
1.057 

1.036 
1.043 

1.046 
1.046 

1.049 
1.046 

1.045 
1.045 
1.046 

1.060 
1.061 
1.063 
1.060 
1.052 

1.067 
1.069 

1.050 
1.051 

1.058 
1.051 

1.058 
1.057 

1.059 
1.061 

1.084 
1.087 

1.070 
1.083 

1.060 
1.057 

1.061 
1.057 

1.061 
1.060 

1.066 
1.067 
1.066 
1.076 
1.078 
1.076 
1.086 
1.084 
1.082 
1.093 
1.093 
1.093 

1.063 
1.068 

1.094 
1.093 

1.084 
1.086 

1.070 
1.072 

1.070 
1.072 

1.067 
1.067 

1.044 
1.037 

1.114 
1.111 

1.102 
1.106 

1.077 
1.077 

1.090 
1.087 

1.085 
1.085 

1.051 
1.060 

1.132 
1.139 
1.109 

1.100 
1.106 

1.092 
1.090 

1.081 
1.086 

1.084 
1.090 

*Not  corrected;  dilution  unknown. 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


Table  38  is  a  condensation  of  Sections  A  and  B  of  Table  37.  Section 
D  gives,  for  each  of  the  ten  concentrations  of  solution,  the  mean  observed 
osmotic  pressure.  Section  E  gives,  for  each  concentration  of  solution, 
the  mean  of  the  corrected  osmotic  pressures. 

TABLE  38. — Cane  sugar,  Series  I  to  VIII. 
SECTION  D.  MEAN  OBSERVED  OSMOTIC  PRESSURES. 


Series  I. 

Series  II. 

Series  III. 

Series  IV. 

Series  V. 

Series  VI. 

Series  VII. 

Series  VIII. 

Cone. 

17°-25°. 

20°-24°. 

0.12°-0.38°. 

4°-5°. 

10°. 

15°. 

25°. 

20°. 

0.1 

2.41 

2.60 

2.42 

2.40 

2.44 

2.48 

2.56 

2.53 

0.2 

4.80 

4.82 

4.78 

4.75 

4.82 

4.91 

5.10 

5.03 

0.3 

7.23 

7.60 

7.10 

7.07 

7.19 

7.33 

7.57 

7.45 

0.4 

9.62 

9.74 

9.35 

9.43 

9.57 

9.78 

10.12 

9.96 

0.5 

12.10 

12.35 

11.77 

11.82 

11.99 

12.29 

12.73 

12.49 

0.6 

14.46 

14.96 

14.12 

14.43 

14.54 

14.86 

15.42 

15.20 

0.7 

16.91 

17.35 

16.68 

16.79 

17.09 

17.39 

18.03 

17.84 

0.8 

19.47 

19.80 

19.15 

19.31 

19.75 

20.09 

20.73 

20.60 

0.9 

21.89 

22.29 

21.89 

22.15 

22.28 

22.94 

23.69 

23.31 

1.0 

24.56 

24.81 

24.45 

24.53 

25.06 

25.42 

26.36 

26.12 

SECTION  E.  MEAN  CORRECTED  OSMOTIC  PRESSURES. 

0.1 

(*) 

2.71 

2.44 

2.42 

2.44 

2.48 

2.56 

2.53 

0.2 

(*) 

4.94 

4.81 

4.79 

4.82 

4.92 

5.10 

5.03 

0.3 

(*) 

7.41 

7.16 

7.13 

7.19 

7.33 

7.57 

7.45 

0.4 

(*) 

9.95 

9.44 

9.55 

9.57 

9.78 

10.12 

9.96 

0.5 

(*) 

12.63 

11.93 

11.97 

12.00 

12.29 

12.73 

12.49 

0.6 

(*) 

15.44 

14.38 

14.56 

14.54 

14.86 

15.42 

15.20 

0.7 

(*) 

17.89 

16.94 

17.05 

17.11 

17.41 

18.05 

17.84 

0.8 

(*) 

20.63 

19.49 

19.63 

19.79 

20.13 

20.76 

20.60 

0.9 

(*) 

22.75 

22.32 

22.52 

22.37 

23.01 

23.75 

23.31 

1.0 

(*) 

25.59 

25.10 

24.97 

25.21 

25.45 

26.42 

26.12 

*Not  corrected;  dilution  unknown. 
TABLE  39. — Mean  ratio  of  corrected  osmotic  to  calculated  gas  pressure. 


Series. 

Tempera- 
ture. 

Concentration. 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1.0 

degrees. 

I 

17  to  25 

(Series  I  not  corrected.     Dilution  unknown.) 

II 

20       24 

1.119 

1.025 

1.031 

1.034 

1.045 

1.061 

1.056 

1.066 

1.041 

1.056 

III 

0.12       0.38 

1.093 

1.077 

1.071 

1.059 

1.071 

1.076 

1.086 

1.094 

1.113 

1.127 

IV 

4       5 

1.066 

1.053 

1.051 

1.055 

1.057 

1.068 

1.077 

1.085 

1.104 

1.103 

Means  (I-IV)  .  .  . 

(1.093) 

(1.052) 

(1.051) 

(1.049) 

(1.058) 

(1.068) 

(1.073) 

(1.082) 

(1.086) 

(1.095) 

V 

10 

1.054 

1.043 

1.038 

1.036 

1.040 

1.051 

1.058 

1.071 

1.077 

1.091 

VI 

15 

1.054 

1.047 

1.040 

1.040 

1.046 

1.055 

1.059 

1.071 

1.089 

1.083 

VIII 

25 

1.057 

1.050 

1.039 

1.042 

1.045 

1.060 

1.066 

1.077 

1.084 

1.093 

VII 

20 

1.053 

1.048 

1.038 

1.041 

1.048 

1.058 

1.060 

1.067 

1.085 

1.087 

Means  (V-VIII)  . 

(1.055) 

(1.047) 

(1.039) 

(1.040) 

(1.045) 

(1.056) 

(1.061) 

(1.072) 

(1.084) 

(1.089) 

Table  39  gives,  for  each  concentration  of  solution,  the  mean  of  the 
ratios  of  the  corrected  osmotic  pressures  to  calculated  gas  pressures.  It 
is  divided  into  two  groups  of  four  series  each;  and  from  the  first  of  these 


CANE   SUGAR.  149 

the  data  pertaining  to  Series  I  are  omitted  because  the  extent  of  the 
dilution  in  that  series  is  unknown.  Throughout  the  three  remaining 
series  of  the  first  division  the  means  of  maintaining  temperature  were 
very  imperfect,  and  the  dilution  of  the  cell  contents,  as  determined  by 
the  loss  in  rotation,  was  large.  These  unsatisfactory  conditions  are 
reflected  in  the  large  variations  in  the  ratios  obtained  at  different 
temperatures  for  the  individual  concentrations  of  solution — that  is,  in 
the  ratios  which  are  placed  in  the  several  vertical  columns. 

Throughout  the  series  of  the  second  group,  on  the  other  hand,  the 
temperatures  maintained  were  constant,  or  very  nearly  so,  and  there 
was  very  little  or  no  loss  in  concentration.  The  better  conditions  under 
which  V  to  VIII  were  carried  out  are  likewise  reflected  in  the  closer 
agreement  of  the  ratios  in  the  several  vertical  columns — the  mean 
variation  for  all  concentrations  being  0.007,  and  the  largest  for  any 
single  concentration,  0.012.  It  is  clear  that  any  conclusions  which 
may  be  drawn  from  the  relations  found  in  the  table  should  be  based 
upon  the  data  in  the  second  group  only.  An  inspection  of  these  will 
show  that: 

1.  The  mean  ratios  of  osmotic  to  gas  pressure  for  every  concentration 
of  solution,  as  well  as  all  the  individual  ratios,  are  considerably  above 
unity.     This  is  also  true  throughout  the  first  group.    The  observation 
that  between  0°  and  25°,  the  osmotic  pressure  of  cane-sugar  solutions 
is  considerably  higher  than  the  calculated  gas  pressure  of  the  solute  has 
been  amply  confirmed  by  later  measurements.     It  is  not  necessary,  at 
the  present  time,  to  search  for  an  explanation  of  this  excessive  osmotic 
pressure,  but  the  fact  that  all  ratios  have  been  found  to  become  unity 
at  high  temperatures  suggests  a  concentration  of  the  solutions  through 
hydration. 

2.  The  ratios,  from  concentration  to  concentration,  are  irregular, 
but,  in  general,  they  diminish  from  the  0.1  weight-normal  solution, 
then  show  a  tendency  to  become  constant  through  the  0.2,  0.3,  and 
0.4  concentrations,  and  finally  they  rise  again  continuously  through  the 
0.5  and  all  succeeding  concentrations.     The  general  trend  is  obviously 
as  stated,  though  it  is  somewhat  confusing  in  its  details.     Later  investi- 
gations have  shown  that  the  ratio  is  relatively  high  in  the  0.1  solution; 
markedly  lower,  but  constant,  through  the  0.2, 0.3,  and  0.4;  higher  again 
in  the  0.5,  and  still  higher  in  each  succeeding  concentration.    This  lack 
of  constancy  of  ratio  from  concentration  to  concentration  suggests,  but 
does  not  prove,  that  the  osmotic  pressures  of  cane-sugar  solutions  do  not 
conform  to  the  law  of  Boyle. 

3.  The  ratios  at  different  temperatures  are  fairly  constant  for  each 
concentration.     Constancy  in  this  respect  is  a  test  of  conformity  to 
the  law  of  Gay-Lussac.     It  will  be  shown  later  that,  between  0°  and 
25°,  all  solutions  of  cane  sugar  ranging  in  concentration  from  0.1  to 
1.0  weight-normal  do  obey  this  law. 


CHAPTER  VII. 

GLUCOSE. 
PRELIMINARY  DETERMINATIONS  OF  OSMOTIC  PRESSURE. 

The  three  series  of  measurements  of  the  osmotic  pressure  of  glucose, 
which  are  to  be  reported  in  the  present  chapter,  were  each  made 
concurrently  with  one  or  another  of  the  eight  preliminary  series  on 
cane  sugar,  which  have  been  described  in  Chapter  VI.  Their  principal 
purpose,  like  that  of  the  earlier  work  upon  cane  sugar,  was  the  devel- 
opment of  the  method. 

It  was  apprehended  that  greater  difficulty  would  be  experienced  in 
securing  solute-proof  membranes  for  glucose  than  for  cane  sugar,  and 
such  was  found  to  be  the  case.  It  was  not  possible  to  decide,  however, 
whether  this  was  due  to  the  easier  penetration  of  the  membranes  by 
glucose,  or  to  the  fact  that  the  membranes  in  cells  containing  glucose 
were  (apparently)  much  more  vigorously  attacked  by  penicillium  than 
those  in  cells  containing  cane  sugar. 

The  manipulation  and  the  facilities  for  the  maintenance  of  tempera- 
ture were  precisely  the  same  for  glucose  as  for  the  cotemporary  work 
upon  cane  sugar;  and,  since  these  have  been  fully  described  in  connection 
with  the  latter,  it  will  be  necessary  only  to  designate  the  chronological 
parallelisms  of  the  work  upon  the  two  substances. 

SERIES  I.* 

Series  I  (for  glucose)  and  Series  II  (for  cane  sugar)  were  carried  out 
during  the  same  year  and  under  the  same  conditions. 

The  material  employed  was  the  so-called  "  Traubenzucker  Kahlbaum" 
It  was  pulverized  and  freely  aerated  over  calcium  chloride  by  means  of 
a  current  of  dried  air,  in  order  to  hasten  the  removal  of  the  odor  of 
alcohol.  After  this  treatment,  the  material  did  not  sensibly  lose  in 
weight  when  heated  to  a  higher  temperature  in  an  air-bath.  It  melted 
quite  sharply  at  146°.  Two  determinations  of  carbon  and  hydrogen 
gave  40.03  and  40.04  instead  of  39.98  per  cent  for  the  former,  and  6.48 
and  6.83  instead  of  6.71  per  cent  for  the  latter. 

The  penicillium  was  not  under  good  control  at  this  time  and  its 
attacks  upon  the  membranes  were  persistent  and  destructive  through- 
out the  whole  series.  Without  doubt  the  results  suffered  somewhat, 
in  point  of  accuracy,  on  that  account. 

*  Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer  and  B.  F.  Lovelace.  Am.  Chem.  Jour., 
xxxvii,  324. 

151 


152 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


The  data  of  Series  I  are  given,  in  a  condensed  form,  in  Table  40,  under 
the  following  heads:  Extreme  bath  temperature,  mean  bath  tempera- 
ture, percentage  loss  in  rotation,  osmotic  pressure  corrected  for  dilution, 
calculated  gas  pressure  of  solute,  and  ratio  of  corrected  osmotic  to 
calculated  gas  pressure. 

TABLE  4<X — Glucose,  Series  I. 


Concentration. 

Extreme 
temperature. 

Mean 
temperature. 

Loss  in 
rotation. 

Corrected 
osmotic 
presstire. 

Calculated 
gas 
pressure. 

Ratio. 

degrees. 

degrees. 

p.  ct. 

0.1 

23.  90  to  24.  40 

24.10 

3.70 

2.39 

2.42 

0.988 

" 

25.00       25.20 

25.10 

0.00 

2.42 

2.43 

0.996 

0.2 

24.00       24.20 

24.10 

0.94 

4.76 

4.85 

0.981 

" 

24.70       25.20 

24.93 

0.94 

4.77 

4.86 

0.981 

0.3 

22.20       22.30 

22.20 

0.00 

7.12 

7.22 

0.986 

" 

23.30       23.60 

23.48 

0.64 

7.17 

7.25 

0.989 

0.4 

26.80       27.00 

26.90 

0.48 

9.70 

9.78 

0.992 

" 

26.60       26.70 

26.60 

0.48 

9.65 

9.77 

0.988 

0.5 

21.75       21.90 

21.86 

1.94 

12.07 

12.03 

1.003 

" 

23.80       24.50 

24.17 

1.16 

12.00 

12.12 

0.990 

0.6 

23.30       22.70 

22.57 

3.76 

14.56 

14.46 

1.007 

" 

22.35       22.45 

22.40 

1.96 

14.32 

14.40 

0.994 

" 

22.30       22.40 

22.30 

0.32 

14.29 

14.45 

0.997 

0.7 

22.20       22.32 

22.26 

2.54 

16.82 

16.85 

0.998 

" 

25.30       25.60 

25.43 

2.82 

16.96 

17.04 

0.996 

" 

22.70       22.70 

22.70 

1.69 

16.75 

16.88 

0.992 

0.8 

23.00       23.00 

23.00 

2.76 

19.27 

19.31 

0.998 

" 

23.10       23.50 

23.28 

1.74 

19.16 

19.33 

0.991 

" 

23.60       23.70 

23.64 

1.74 

19.25 

19.35 

0.993 

0.9 

23.70       24.10 

23.80 

1.45 

21.64 

21.80 

0.993 

" 

22.50       22.70 

22.58 

1.00 

21.49 

21.70 

0.990 

41 

23.00       23.10 

23.10 

0.66 

21.63 

21.74 

0.995 

1.0 

22.00       22.30 

22.20 

0.91 

24.12 

24.08 

1.002 

11 

22.60       22.70 

22.60 

0.91 

24.00 

24.11 

0.995 

" 

22.10       22.10 

22.10 

1.72 

24.03 

24.07 

0.998 

Mean 

=  0.994 

Table  41  gives  the  extreme  variations  in  temperature  for  each  experi- 
ment. 

TABLE  41. — Glucose,  Series  I.     Fluctuations  in  bath  temperature. 


Concen- 
tration. 

Varia- 
tion. 

Concen- 
tration. 

Varia- 
tion. 

Concen- 
tration. 

Varia- 
tion. 

degrees. 

degrees. 

degrees. 

0.1 

0.50 

0.5 

0.15 

0.8 

0.00 

" 

0.20 

" 

0.70 

" 

0.40 

0.2 

0.20 

0.6 

0.40 

" 

0  10 

" 

0.50 

" 

0.10 

0.9 

0.40 

0.3 

0.10 

" 

0.10 

" 

0.20 

" 

0.30 

0.7 

0.12 

" 

0.10 

0.4 

0.20 

" 

0.30 

1.0 

0.30 

" 

0.10 

" 

0.00 

" 

0.10 

" 

0.00 

Sum  =  5.57 

Mean  =  0.22 

GLUCOSE.  153 

The  sum  of  the  variations  in  bath  temperature  was  5.57°  and  the 
mean  was  0.22°.  The  corresponding  values  for  the  parallel  cane-sugar 
series  (II)  were  4.35°  and  0.21°,  which  shows  that  the  success  attained 
in  maintaining  temperature  was  about  the  same  in  glucose  Series  I  as 
in  cane-sugar  Series  II. 

The  sum  of  the  rotations  of  all  the  solutions  used  in  glucose  Series  I 
was  758.85°.  The  sum  of  all  the  losses  was  8.60°  or  1.13  per  cent.  Ths 
dilution  in  the  companion  cane-sugar  series  was  2.86  per  cent,  or  2.53 
times  as  large  as  in  the  case  of  glucose. 

The  observed  osmotic  pressures  have  been  corrected  for  all  of  the  loss 
in  rotation,  though,  as  explained  in  the  preceding  chapter,  the  dilution 
which  occurs  when  the  cells  are  opened,  if  known,  should  be  deducted. 
But,  since  the  total  dilution  was  only  1.13  per  cent,  and  since  certainly 
kss  than  half  of  it  occurred  when  the  cells  were  opened,  the  results  do 
not  greatly  suffer  by  the  inclusion  of  the  latter. 

The  striking  features  of  Table  40  will  be  found  in  the  last  column, 
in  which  are  given  the  ratios  of  osmotic  to  the  calculated  gas  pressures 
of  the  solute.  Considering  the  still  undeveloped  condition  of  the 
method  by  which  they  were  obtained,  these  ratios  are  remarkably 
uniform  throughout  the  whole  series.  The  mean  of  all  of  them  is  0.994, 
and  the  greatest  divergences  from  this  mean  are  +0.013  and  —0.013. 
It  will  be  recalled  in  this  connection  that,  in  the  case  of  cane  sugar, 
the  ratios  of  osmotic  to  gas  pressure  varied  considerably  from  concen- 
tration to  concentration.  The  second  noteworthy  feature  of  these 
ratios  is  that  they  approach  unity — quite  as  closely  probably  as  the 
defects  of  the  method  at  that  time  could  be  expected  to  permit.  If 
the  approximate  correctness  of  the  pressures  given  in  Table  40  is  estab- 
lished by  later  investigations,  it  will  mean  that,  within  the  range  of 
temperatures  22°  to  25°,  the  osmotic  pressure  of  glucose  solutions 
obeys  the  laws  both  of  Boyle  and  Gay-Lussac,  since  that  is  the  only 
interpretation  of  the  unit  ratios  of  osmotic  to  gas  pressure.  It  is  not 
yet  known  whether  this  ratio  will  be  confirmed  for  the  temperatures 
in  question,  since  the  work  at  25°  has  not  been  repeated  under  condi- 
tions insuring  precision.  It  is  already  known,  however,  that  at  30°, 
40° ,  and  50°  the  ratio  of  osmotic  to  gas  pressure  is  unity  for  solutions 
of  glucose. 

The  molecular  weight  for  glucose  which  is  derived  from  the  mean 
ratio  0.994 — under  the  assumption  that  osmotic  pressure  obeys  the 
laws  of  Boyle  and  Gay-Lussac — is  179.82  instead  of  178.74. 

In  the  case  of  cane  sugar,  Series  I — without  correction — gave  a  molec- 
ular weight  of  341.41  (0  =  16)  instead  of  342.22;  while  Series  II— after 
correction  for  the  loss  in  rotation  as  inversion — gave  a  molecular  weight 
of  337.59  (H  =  1)  instead  of  339.60.  The  excellent  molecular  weight 
which  was  legitimately  derived  from  the  results  in  glucose  Series  I  was 
partly  responsible  for  the  pertinacity  with  which,  for  a  time,  the  mis- 


154      OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

taken  views  regarding  cane-sugar  Series  I  and  II  were  held — the  views, 
namely,  (1)  that,  not  having  found  much  invert  sugar  in  Series  I  by 
the  method  of  Fehling,  the  solutions  had  maintained  their  concentra- 
tion; and  (2)  that,  having  found  considerable  loss  in  rotation  in  Series 
II,  it  was  due  to  inversion  caused  by  penicillium.  The  early  errors  of 
interpretation  regarding  cane-sugar  Series  I  and  II  have  been  corrected 
in  the  preceding  chapter;  but,  up  to  the  present  time,  no  good  reason 
has  appeared  for  questioning  the  general  correctness  of  the  results  of 
glucose  Series  I  as  they  are  presented  in  Table  40. 

SERIES  II.* 

Glucose  Series  II  and  cane-sugar  Series  III  were  carried  out  at 
about  the  same  time  and  under  identical  conditions.  It  was  sought 
in  both  to  maintain  a  temperature  as  close  as  possible  to  0°.  The 
means  which  were  employed  for  this  purpose  have  been  described  in 
connection  with  the  account  which  was  given  of  the  work  in  cane  sugar 
Series  III. 

The  material  used  was  Traubenzucker  Kahlhaum,  but  in  the  begin- 
ning it  was  distinctly  less  pure  than  that  employed  in  Series  I.  After 
aerating  the  pulverized  substance  and  allowing  it  to  stand  in  an 
exhausted  desiccator  until  it  gave  no  reaction  for  alcohol,  it  was 
found  to  have  a  somewhat  uncertain  melting-point  of  143°.  Four 
determinations  of  carbon  and  hydrogen  gave:  for  the  former,  40.28, 
40.26,  40.35,  and  40.35  instead  of  39.98  per  cent;  and  for  the  latter, 
6.60,  6.66,  6.62,  and  6.69  instead  of  6.71  per  cent.  A  solution,  con- 
taining 32.65  grams  of  the  glucose  in  100  cubic  centimeters  at  17.5°, 
gave  a  rotation  of  101.45  instead  of  100  saccharimetric  degrees.  Before 
using  the  material  for  the  determination  of  osmotic  pressure,  it  was  four 
times  recrystallized  by  precipitation  from  aqueous  solution  by  alcohol. 
Thus  purified,  its  melting-point  was  found  to  be  145°  to  146°  instead  of 
146°,  and  the  standard  solution  gave  a  rotation  of  100.5  saccharimetric 
degrees.  Two  analyses  gave :  for  carbon,  40.09  and  39.96  instead  of  39.98 
per  cent;  and  for  hydrogen,  6.64  and  6.77  instead  of  6.71  per  cent. 

The  sum  of  all  the  fluctuations  in  bath  temperature  was  1.47°  and 
the  mean  was  0.07°.  In  the  parallel  cane-sugar  series  (III)  the  sum 
was  2.81°,  and  the  mean  was  0.10°. 

The  sum  of  the  rotation  of  all  the  solutions  of  glucose  Series  II  was 
552.90°.  The  sum  of  all  the  losses  in  rotation  was  5.84°,  or  1 .06  per  cent. 
In  the  companion  cane-sugar  series,  the  percentage  loss  in  rotation  was 
1.73  per  cent.  The  dilution  in  the  glucose  series  was,  therefore,  less  by 
0.67  per  cent  than  in  that  of  cane  sugar. 

Except  in  the  case  of  the  0.1  normal  solution,  the  ratios  of  osmotic 
to  gas  pressure  are  quite  uniform.  In  this  respect  glucose  Series  II, 

*Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer,  and  F.  M.  Rogers.  Am.  Chem.  Jour.,  xxxvn,  558. 


GLUCOSE. 


155 


like  glucose  Series  I,  differs  strikingly  from  all  of  the  eight  cane-sugar 
series,  in  which  the  ratios  differed  from  concentration  to  concentration. 
The  mean  ratio  for  the  0.1  normal  solution  is  1.076,  while  the  mean  ratio 
for  the  whole  series  is  1.058.  It  would  be  premature  to  discuss  this 
apparent  exception  at  the  present  time,  but  it  may  be  noted  in  passing 
that  a  similar  increase  in  the  osmotic  pressure  of  very  dilute  solutions, 
when  near  their  freezing-points,  has  been  observed  in  the  case  of  cane 
sugar. 

TABLE  42. — Glucose,  Series  II. 


Concentration. 

Extreme 
temperature. 

Mean 
temperature. 

Loss  in 
rotation. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

degree. 

degree. 

p.  ct. 

0.1 

0.26  to  0.38 

0.28 

0.38 

2.40 

2.23 

1.074 

" 

0.18      0.28 

0.24 

0.00 

2.40 

" 

1.077 

0.2 

0.10      0.12 

0.12 

0.96 

4.66 

4.45 

1.047 

" 

0.12       0.14 

0.13 

0.48 

4.68 

" 

1.051 

0.3 

0.16       0.24 

0.19 

0.64 

7.04 

6.68 

1.054 

" 

0.17       0.26 

0.26 

0.71 

7.04 

" 

1.054 

0.4 

0.12       0.14 

0.13 

0.73 

9.35 

8.91 

1.049 

" 

0.14       0.30 

0.21 

0.87 

9.33 

" 

1.047 

0.5 

0.12       0.19 

0.17 

0.58 

11.69 

11.14 

1.050 

" 

0.14       0.29 

0.24 

0.98 

11.69 

" 

1.049 

0.6 

0.08       0.14 

0.12 

1.47 

14.12 

13.36 

1.057 

" 

0.08       0.08 

0.08 

1.47 

14.12 

" 

1.057 

0.7 

0.06       0.10 

0.08 

0.99 

16.44 

15.59 

1.055 

" 

0.06       0.06 

0.06 

0.57 

16.42 

" 

1.053 

0.8 

0.12       0.15 

0.13 

1.13 

18.86 

17.82 

1.058 

" 

0.12       0.15 

0.13 

0.88 

18.86 

" 

1.058 

0.9 

0.10       0.24 

0.16 

1.58 

21.37 

20.05 

1.066 

" 

0.10      0.24 

0.15 

0.91 

21.40 

11 

1.067 

1.0 

0.12       0.22 

0.17 

1.43 

23.77 

22.28 

1.067 

" 

0.12       0.22 

0.17 

1.20 

23.72 

" 

1.064 

Mean 

=  1.058 

The  most  noteworthy  feature  of  the  ratios  is  their  high  value  as 
compared  with  the  corresponding  ratios  of  glucose  Series  I.  The  mean 
ratios  of  the  two  series  are  0.994  and  1.058  respectively.  The  difference 
between  them  is  about  6  per  cent.  The  only  essential  difference  in 
the  conditions  under  which  the  two  series  were  carried  out  was  that 
of  temperature,  Series  I  having  been  done  at  approximately  25°  and 
Series  II  at  approximately  0°.  The  decrease  in  ratio  with  rise  in 
temperature  suggests  a  hydration  of  the  solute  at  lower  temperatures, 
which  diminishes  or  disappears  when  the  temperature  is  raised.  But 
this  matter  can  be  discussed  more  advantageously  when  more  facts 
concerning  glucose  have  been  established,  and  in  connection  with  similar 
conduct  on  the  part  of  cane-sugar  solutions. 


156 


OSMOTIC   PRESSURE   OF   AQUEOUS   SOLUTIONS. 


SERIES  III.* 

Glucose  III  and  cane  sugar  V  were  parallel  series.  Before  they  were 
undertaken,  the  means  of  maintaining  temperature  and  the  manipula- 
tion concerned  in  the  closing  and  opening  of  the  cells  had  been  greatly 
improved,  with  corresponding  reduction  in  temperature  fluctuations 
and  in  dilution  of  the  cell  contents. 

The  material  employed  in  Series  III  was  the  same  as  in  glucose 
Series  II. 

TABLE  43. — Glucose,  Series  HI. 


Concentration. 

Extreme 
temperature. 

Mean 
temperature. 

Loss  in 
rotation. 

Corrected 
osmotic 
pressure. 

Calculated 
gas 
pressure. 

Ratio. 

degrees. 

degrees. 

p.  ct. 

0.1 

10.10  to  10.10 

10.10 

0.96 

2.38 

2.31 

1.036 

" 

10.20       10.20 

10.20 

0.00 

2.39 

" 

1.034 

0.2 

10.40       10.40 

10.40 

0.00 

4.78 

4.63 

1.032 

" 

10.20       10.20 

10.20 

0.00 

4.74 

4.61 

1.028 

0.3 

10.00       10.00 

10.00 

0.00 

7.11 

6.92 

1.027 

0.4 

10.10       10.20 

10.15 

0.00 

9.50 

9.24 

1.028 

" 

10.10       10.20 

10.15 

0.00 

9.54 

" 

1.032 

0.5 

10.05       10.30 

10.18 

0.00 

11.91 

11.55 

1.032 

" 

10.00       10.20 

10.10 

0.00 

11.90 

11.54 

1.031 

0.6 

10.00       10.20 

10.10 

0.00 

14.30 

13.85 

1.032 

" 

10.00       10.00 

10.00 

0.00 

14.31 

13.84 

1.034 

0.7 

10.00       10.10 

10.05 

0.56 

16.70 

16.16 

1.033 

" 

10.00       10.10 

10.05 

0.00 

16.69 

" 

1.033 

0.8 

10.00       10.15 

10.08 

0.25 

19.04 

18.46 

1.031 

" 

10.00       10.00 

10.00 

0.00 

19.05 

" 

1.032 

0.9 

10.00       10.20 

10.10 

0.34 

21.39 

20.78 

1..036 

" 

10.00       10.00 

10.00 

0.45 

21.38 

20.77 

1.029 

1.0 

10.00       10.10 

10.05 

0.30 

23.79 

23.08 

1.031 

" 

10.00       10.10 

10.05 

0.41 

23.80 

** 

1.031 

Mean 

=  1.031 

The  sum  of  all  fluctuations  in  bath  temperature  was  1.60°  and  the 
mean  variation  was  0.08°.  In  the  companion  cane-sugar  series,  the 
mean  variation  was  also  0.08°. 

The  sum  of  the  rotations  of  all  the  solutions  employed  in  glucose 
Series  III  was  541.70°,  and  the  total  loss  was  1.05°,  or  0.20  per  cent. 
The  loss  in  the  corresponding  cane-sugar  series  was  also  0.20  per  cent. 
The  decline  of  the  dilution  from  1.06  per  cent  in  glucose  Series  II  to 
0.20  per  cent  in  Series  III  is  a  fair  measure  of  the  improvement  which 
had  been  made  in  the  manipulation  of  the  cells.  The  decline  in  dilu- 
tion in  the  case  of  the  parallel  cane-sugar  series  was  from  1.73  to  0.20 
per  cent. 

The  ratios  of  osmotic  to  gas  pressure  in  Series  III  are  even  more 
uniform  throughout  the  whole  range  of  concentration  than  are  those 
of  Series  I  and  II.  This  uniformity  of  ratio,  which  is  characteristic  of 

*Measurements  by  H.  N.  Morse  and  W.  W.  Holland.    Am.  Chem.  Jour.,  XL,  1. 


GLUCOSE. 


157 


glucose  solutions,  but  not  of  solutions  of  cane  sugar — except  at  com- 
paratively high  temperatures — appears  to  signify  that  the  osmotic 
pressure  of  glucose  obeys  the  law  of  Boyle.  Perfect  uniformity  of  ratio 
at  a  given  temperature  means,  of  course,  that  the  osmotic  pressures  are 
proportional  to  the  concentration  of  the  solutions,  which  is  the  form 
of  Boyle's  law  as  applied  to  solutions.  But  any  extended  discussion 
of  this  subject  at  the  present  time  would  be  premature. 

The  essential  facts  connected  with  glucose  Series  I  to  III  are  sum- 
marized in  Tables  44,  45,  and  45a. 

TABLE  44. — Parallel  series  of  glucose  and  cane  sugar. 


Glucose 
I. 

Cane  sugar 
II. 

Glucose 
II. 

Cane  sugar 
III. 

Glucose 
III. 

Cane  sugar 
V. 

1.  Mean  variation  in  bath 
temperature  

degrees. 
0.22 

degrees. 
0.21 

degrees. 
0.07 

degrees. 
0.10 

degrees. 
0.08 

degrees 
0.08 

2.  Percentage  dilution  

1.13 

2.86 

1.06 

1.73 

0.20 

0  20 

TABLE  45. — Glucose,  Series  I  to  III. 


Concentration. 

Extreme  variations  in  bath  temperature. 

Loss  in  rotation. 

Series  I. 

Series  II. 

Series  III. 

Series  I. 

Series  II. 

Series  III. 

degrees. 

degrees. 

degrees. 

degrees. 

degrees. 

degrees. 

0.1 

0.50 

0.12 

0.00 

0.20 

0.02 

0.05 

" 

0.20 

0.10 

0.00 

0.00 

0.00 

0.00 

0.2 

0.20 

0.02 

0.00 

0.10 

0.10 

0.00 

" 

0.50 

0.02 

0.00 

0.10 

0.05 

0.00 

0.3 

0.10 

0.08 

0.00 

0.00 

0.10 

0.00 

11 

0.30 

0.09 

0.10 

0.11 

0.4 

0.20 

0.02 

0.10 

0.10 

0.15 

0.00 

" 

0.10 

0.16 

0.10 

0.10 

0.18 

0.00 

0.5 

0.15 

0.07 

0.25 

0.50 

0.15 

0.00 

" 

0.70 

0.15 

0.20 

0.30 

0.25 

0.00 

0.6 

0.40 

0.06 

0.20 

0.15 

0.55 

0.00 

" 

0.10 

0.00 

0.00 

0.60 

0.45 

0.00 

" 

0.10 

0.10 

0.7 

0.12 

0.04 

o.io 

0.90 

6.35 

0.20 

" 

0.30 

0.00 

0.10 

1.00 

0.20 

0.00 

" 

0.00 

0.60 

0.8 

0.00 

0.03 

0.15 

1.10 

6.45 

0.10 

» 

0.40 

0.03 

0.00 

0.70 

0.35 

0.00 

" 

0.10 

0.70 

0.9 

0.40 

0.14 

0.20 

0.65 

6.70 

0.15 

" 

0.20 

0.14 

0.00 

0.45 

0.40 

0.20 

" 

0.10 

O.SO 

1.0 

0.30 

0.10                 0.10 

0.45 

6.70 

0.15 

" 

0.10 

0.10 

0.10 

0.45 

0.58 

0.20 

" 

0.00                  

0.85 

Means 

0.22 

0.07                 0.08 

1.13 

1.06 

0.20 

158  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 

TABLE  45a. — Glucose,  Series  I  to  III. 


Mean  osmotic  pressures. 

Mean  ratios  of  osmotic  to  gas 

Concen- 

tration. 

Series  I. 

Series  II. 

Series  III. 

Series  I. 

Series  II. 

Series  III. 

22°-25° 

0.06°-0.38° 

10.00°-10.40° 

22°-25° 

0.06°-0.38° 

10.00°-10.40° 

0.1 

2.40 

2.40 

2.39 

0.992 

1.076 

1.035 

0.2 

4.77 

4.67 

4.76 

0.981 

1.049 

1.030 

0.3 

7.15 

7.04 

7.11 

0.988 

1.054 

1.027 

0.4 

9.68 

9.34 

9.52 

0.990 

1.048 

1.030 

0.5 

10.04 

11.69 

11.91 

0.997 

1.050 

1.032 

0.6 

14.39 

14.12 

14.31 

0.999 

1.057 

1.033 

0.7 

16.84 

16.43 

16.70 

0.995 

1.054 

1.033 

0.8 

19.23 

18.86 

19.05 

0.994 

1.058 

1.032 

0.9 

21.59 

21.39 

21.39 

0.993 

1.067 

1.030 

1.0 

24.05 

23.75 

23.80 

0.998 

1.066 

1.031 

Means 

0.994 

1.058 

1.031 

CHAPTER  VIII. 

CANE  SUGAR. 

FINAL  DETERMINATIONS  OF  OSMOTIC  PRESSURE. 

The  determinations  of  osmotic  pressure  which  were  made  after  the 
method  had  been  perfected  as  described  in  Chapters  VI  and  VII  are 
designated  as  "final,"  because  they  are  believed  to  be  in  a  high  degree  reli- 
able. It  is  characteristic  of  them  all  that  there  was  neither  any  material 
variation  in  bath  temperature  during  any  experiment,  nor  any  dilution  of 
the  cell  contents  which  could  be  detected  by  the  polariscope.  It  is  not 
meant  thereby  that  we  were  able  to  maintain  absolutely  constant  tem- 
peratures in  the  large  baths  which  were  employed.  There  were  frequent 
fluctuations  which  amounted  sometimes  to  0.02°,  but  usually  to  not  more 
than  0.01°.  If  the  variation  in  bath  temperature  did  not  exceed  0.02° 
during  an  experiment,  it  was  considered  to  have  remained  sufficiently 
constant.  Occasionally  accidents  happened  to  the  regulating  devices, 
and  the  baths  were  temporarily  thrown  "off  temperature"  in  consequence. 
If  the  difficulty  was  soon  discovered  and  quickly  remedied,  the  resulting 
fluctuation  in  bath  temperature  was  small,  and  the  experiment  was  saved 
by  discarding  all  readings  of  pressure  until  the  cell  contents  had  had 
ample  time  to  recover  from  any  thermometer  effects  due  to  the  accident. 
If  the  trouble  occurred  during  the  night  and  was  not,  therefore,  dis- 
covered until  the  temperature  of  the  bath  had  risen  or  fallen  a  consider- 
able fraction  of  a  degree,  the  determination  was  usually  discarded.  The 
most  frequent  cause  of  difficulty  with  the  regulating  devices  was  a  tem- 
porary interruption  of  the  main  current  at  its  source,  i.  e.,  at  the  power 
house. 

The  sugar  which  was  employed  for  the  "preliminary"  determinations 
described  in  Chapters  VI  and  VII  was  "rock  candy,"  which  was  not  puri- 
fied by  recrystallization.  This  material  is  known,  however,  to  contain,  as 
a  rule,  some  mother  liquor  and  to  be  otherwise  impure,  notwithstanding 
its  fine  appearance.  Moreover,  it  had  been  observed  that  the  material 
obtained  from  rock  candy  by  reprecipitation  gave  somewhat  higher  pres- 
sures than  had  been  obtained  with  the  unpurified  sugar.  It  was  decided, 
therefore,  to  subject  the  sugar  which  was  to  be  used  for  the  "final"  meas- 
urements to  a  thorough-going  purification.  The  method  employed  was 
essentially  that  of  Cohen  and  Commelin.*  150  pounds — approximately 
70  kilograms — of  the  best  rock  candy  were  procured  and  subjected  to  the 
treatment  described  below.  Kilogram  quantities  of  it  were  dissolved, 
each  hi  500  c.c.  of  previously  boiled  distilled  water  which,  when  making 

*Zeitschrift  fur  physikalische  Chemie,  LXVI,  1. 

159 


160 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


the  solutions,  was  warmed,  but  not  to  a  temperature  above  60°.  The 
solution  (sometimes  thinned  with  a  little  alcohol)  was  filtered,  and  from 
the  filtrate  the  sugar  was  precipitated  by  alcohol  which  had  been  distilled 
from  lime — a  few  crystals  of  the  purest  sugar  being  used  to  start  the  precip- 
itation. The  precipitated  sugar  was  collected  on  a  perforated  porcelain 
disk  in  the  bottom  of  a  glass  funnel,  and  freed  as  perfectly  as  possible  from 
mother  liquor  by  means  of  the  filter  pump.  The  material  was  then  trans- 
ferred from  the  funnel  to  a  porcelain  dish  and  mixed  to  a  thin  paste  with 
85  per  cent  alcohol.  Finally  it  was  again  filtered,  and  then  nearly  dried 
by  drawing  through  it  filtered  air.  The  original  rock  candy  and  the 
product  of  the  first  crystallization  will  be  designated  hereafter  by  the 
letters  A  and  B.  The  yield  of  B  was  32  kilograms. 

The  various  portions  of  B  were  thoroughly  mixed  and  then  resubjected 
to  the  treatment  which  has  already  been  described,  except  that  the  prod- 
uct of  the  second  precipitation  was  washed  first  with  diluted  ethyl  alco- 
hol and  afterwards  with  warm  methyl  alcohol.  The  yield  of  the  twice 
recrystallized  sugar,  which  will  be  designated  by  the  letter  C,  was  about  16 
kilograms. 

A  portion  of  C  was  again  dissolved,  reprecipitated,  and  washed  with 
both  ethyl  and  methyl  alcohols.  The  product  of  the  third  precipitation 
will  be  designated  by  the  letter  D. 

Combustions  were  made  of  all  four  products,  namely  A,  the  original 
rock  candy;  B,  which  had  been  precipitated  once;  C,  twice;  and  D,  three 
times.  The  results  are  given  below  in  percentages  of  hydrogen  and 
carbon. 

TABLE  46. 


I 

i. 

I 

(. 

C 

] 

3. 

H 

C 

H 

C 

H 

C 

H 

C 

1  

6.432 

42  .  156 

6.436 

42.116 

6.466 

42.151 

6.484 

42  047 

2  

6.495 

42.081 

6.451 

42  .  059 

6.420 

42.081 

6.487 

42  031 

3  

6.477 

42  .  099 

6.465 

42.151 

6.471 

42.116 

6.485 

42  101 

Mean  

6.468 

42.112 

6.451 

42  .  109 

6.452 

42.116 

6.485 

42  060 

Theoretical  .... 

6.481 

42.083 

6.481 

42.083 

6.481 

42.083 

6.481 

42.083 

Differences  .... 

-0.013 

+0.029 

-0.030 

+0.026 

—  0.029 

+0.033 

-0.004 

-0.023 

The  differences  between  the  percentages  of  hydrogen  and  carbon  which 
were  found  and  the  theoretical  values  are  all  within  the  unavoidable  errors 
of  analysis,  and  there  was,  therefore,  no  reason  to  be  discovered  in  the 
figures  given  above  for  regarding  any  one  sample  of  the  sugar  purer  than 
another.  A  determination  of  carbon  and  hydrogen  does  not,  however, 
suffice  for  the  detection  of  glucose  or  invert  sugar  in  cane  sugar;  and 
evidence  of  the  probable  presence  of  reducing  sugars  could  be  discovered 


CANE    SUGAR.  161 

in  all  the  specimens  by  other  means.  Much  time  was  spent  in  attempts  to 
establish  the  limits  within  which  these  might  be  present.  Finally,  how- 
ever, the  whole  question  of  the  purity  of  the  materials  was  referred  to  the 
Bureau  of  Standards  at  Washington.  The  report  which  was  received 
from  the  Bureau  is  given  below. 

Sample  A. — Reducing  substances  in  terms  of  invert  sugar,  0.08 

per  cent  ±  0.005  per  cent. 
Sample  B. — Reducing  substances  in  terms  of  invert  sugar,  0.01 

per  cent  ±  0.005  per  cent. 
Sample  C. — Polarization,  99.93°.     Reducing  substances  in  terms 

of  invert  sugar,  0.01  per  cent  =*=  0.005  per  cent. 
Sample  D. — Polarization,  99.95°.     Reducing  substances  in  terms 

of  invert  sugar,  0.005  per  cent  =*=  0.005  per  cent. 

The  material  employed  for  the  "final"  determinations  of  osmotic  pres- 
sure was  that  designated  by  the  letter  C,  in  which  the  Bureau  of  Standards 
had  found  0.01  per  cent  of  reducing  sugar.  The  sample  D  which  had  been 
three  times  recrystallized  was  doubtless  somewhat  purer,  but  it  was  feared 
that  the  quantity  of  D  in  hand  would  not  suffice  for  all  the  determinations 
which  were  to  be  made,  and  uniformity  of  material  was  of  quite  as  much 
importance  as  absolute  purity. 

The  baths  which  were  devised  for  the  regulation  of  temperature  have 
been  sufficiently  described  in  Chapter  III,  and  it  will  only  be  necessary  to 
explain  in  the  present  chapter  certain  points  as  to  then*  use  in  the  measure- 
ment of  pressure. 

It  has  been  stated  elsewhere  that  the  cells,  whether  in  or  out  of  use, 
are  maintained  at  all  times  at  the  temperature  at  which  they  are  to  be 
employed  for  the  determination  of  pressure.  This  statement  is  correct 
for  all  low  and  moderate  temperatures.  But  when  they  are  to  be  used  at 
high  temperatures,  e.  g.,  above  40°,  it  is  necessary  to  maintain  them  at  a 
temperature  a  little  higher  than  that  at  which  the  measurements  are  to  be 
made,  in  order  to  compensate  the  cooling  effects  of  exposure  while  the 
cells  are  being  filled  and  closed. 

The  same  is  also  true  of  the  solutions.  They  are  made  up  at  the  tem- 
perature of  the  room,  and  then  cooled  or  warmed,  as  the  case  may  require, 
in  closed  flasks,  in  the  baths.  The  baths  which  are  used  for  such  purposes 
are  maintained  at  the  temperature  at  which  measurements  are  to  be  made, 
if  the  temperature  in  question  is  a  low  or  moderate  one;  otherwise,  at  a 
slightly  higher  temperature.  The  amount  of  the  provision  which  is  thus 
made  for  the  cooling  effect  of  exposure  while  filling  the  cells  is  entirely  a 
matter  of  judgment  and  experience.  The  mercury  in  the  manometers  is 
always  at  the  temperature  of  the  room  when  the  cells  are  filled,  and  its 
subsequent  expansion  in  a  bath  of  higher  temperature  must  be  taken  into 
account;  for  this  partially  compensates  any  contraction  of  the  solution 


162       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

when  its  temperature  falls  from  a  higher  level  to  that  of  the  bath.  Hence 
it  is  always  intended,  when  working  at  high  temperatures,  to  have  the  solu- 
tion a  little  too  hot  when  the  cell  goes  into  its  final  bath.  It  is  not  pos- 
sible, however,  to  regulate  the  temperature  conditions  so  perfectly  that, 
after  filling  a  cell  and  introducing  it  into  the  bath,  the  contraction  of  the 
solution  will  exactly  balance  the  expansion  of  the  mercury  in  the  manom- 
eter. For  that  reason  the  cells  are  often  placed  in  a  so-called  "prelimi- 
nary bath"  which  is  more  accessible  and  less  elaborate  than  that  in  which 
the  measurements  of  pressure  are  made;  and  they  are  there  observed  while 
coming  to  temperature.  If  the  observed  pressures  are  considerably 
above  the  approximately  known  osmotic  pressures,  small  portions  of  the 
solutions  are  allowed  to  escape  from  the  cells.  If,  on  the  other  hand,  they 
are  much  below  the  true  osmotic  pressures  for  the  given  temperature,  an 
additional  mechanical  pressure  is  brought  upon  the  contents  of  the  cells. 
When  the  temperature  of  the  cells  and  their  contents  has  finally  reached 
that  of  the  bath,  the  pressures  should  be  very  nearly  equal  to  the  true 
osmotic  pressures  of  the  solutions;  since,  otherwise,  the  inclosed  solutions 
must  suffer  some  concentration  or  dilution.  The  supplementary  process 
of  pressure-adjustment,  described  above,  can  not  be  dispensed  with  in 
high-temperature  work.  At  moderate  and  low  temperatures,  sufficiently 
close  adjustments  of  pressure  can  usually  be  secured  at  the  time  of  closing 
the  cells;  that  is,  the  probable  changes  in  the  volumes  of  the  solutions 
and  of  the  mercury  in  the  manometers  can  be  more  accurately  estimated. 
Nevertheless,  even  at  low  and  moderate  temperatures,  the  cells  are  care- 
fully watched  until  it  is  certain  that  no  further  adjustments  of  pressure 
will  be  necessary  in  order  to  prevent  a  sensible  change  in  the  concentra- 
tion of  the  solutions.  The  pressures  to  which  the  cells  are  adjusted  before 
placing  them  in  the  final  bath,  or  leaving  them  to  come  undisturbed  to 
equilibrium,  are  known  as  "initial"  pressures.  They  are,  of  course,  only 
temporary  values. 

It  has  been  proved  by  a  large  number  of  experiments  that  it  is  imma- 
terial from  which  direction  the  final  equilibrium  pressure  is  approached, 
i.  e.,  whether  from  a  higher  or  lower  initial  pressure.  It  is  only  necessary 
that  the  interval  between  the  initial  and  final  pressures  shall  not  be  sufficient 
to  produce  — through  change  in  the  volume  of  the  cell  contents — a  sensible 
concentration  or  dilution  of  the  inclosed  solution.  In  some  series  of 
measurements,  it  has  been  customary  to  so  adjust  the  initial  pressures  in 
duplicate  determinations  that  the  equilibrium  pressure  was  approached 
in  one  instance  from  above  and  in  the  other  from  below. 

The  importance  of  demonstrating  that  a  solution  has  maintained  its 
original  concentration  throughout  a  measurement  of  pressure  can  not  be 
over-emphasized ;  accordingly,  whenever  a  cell  has  been  filled  and  closed,  a 
part  of  the  solution  has  been  reserved  for  comparison,  with  respect  to 
concentration,  with  the  solution  which  was  removed  from  the  cell  at  the 
close  of  the  experiment.  In  all  the  measurements  recorded  in  the  present 


CANE   SUGAR.  163 

chapter — except  one  which  is  introduced  to  illustrate  concentration  in  the 
cell — the  two  portions  of  the  solutions  were  found  to  have  identical  rota- 
tions. In  other  words,  all  experiments  in  which  the  solutions  were  found 
to  have  suffered  a  change  in  concentration  have  been  discarded.  When- 
ever a  gain  or  loss  in  concentration  has  occurred  in  the  course  of  the  work, 
it  has  usually  been  due  to  a  faulty  adjustment  of  the  initial  pressure,  i.  e., 
the  interval  between  it  and  the  final  pressure  has  been  left  too  large.  The 
osmotic  pressures  of  solutions  whose  concentration  has  changed  in  the 
cells  are  readily  correctible,  if  one  could  only  prove  that  the  cells  have  not 
leaked.  But  the  one  certain  proof  that  no  solute  has  escaped  through 
the  membrane  is  the  fact  that  the  solution  taken  from  the  cell  at  the  close 
of  an  experiment  has  the  same  concentration  as  the  one  which  was  put 
into  it  in  the  beginning.  All  other  demonstrations  of  the  integrity  of  the 
membrane  have  one  or  more  weak  points. 

It  will  be  seen  that  the  possibility  of  a  sensible  dilution  or  concentration 
of  the  solution  in  the  cell  depends  on  the  relation  of  the  nitrogen  volumes 
at  initial  and  at  equilibrium  pressures.  If  the  difference  between  these  is 
very  small  as  compared  with  the  volume  of  the  solution,  there  can  be 
no  material  change  in  concentration.  It  follows  that,  so  far  as  actual 
pressures  are  concerned,  the  preliminary  adjustments  of  pressure  must  be 
much  closer  in  the  case  of  dilute  than  in  that  of  concentrated  solutions; 
moreover,  that  the  difference  between  initial  and  final  pressures  must  be 
made  smaller  when  manometers  of  large  capacity  are  used,  than  when 
those  with  only  moderate  gas  volumes  are  employed.  Since  the  cells  all 
have  a  capacity  of  about  20  c.c.,  it  is  only  necessary,  when  adjusting  the 
initial  pressure,  to  consider  whether  the  subsequent  contraction  or  expan- 
sion of  the  nitrogen  will  constitute  an  appreciable  fraction  of  that  volume. 

The  work  included  in  the  present  chapter  required  three  years  for  its 
completion.  The  number  of  measurements  reported  is  270.  The  average 
rate  of  progress  was,  therefore,  90  determinations  per  year,  or  10  for  each 
working  month.  It  is  to  be  remembered  in  this  connection,  however,  that 
the  labor  required  for  the  mere  measurement  of  osmotic  pressure  is  insig- 
nificant when  compared  with  that  which  must  be  bestowed  upon  the  cells, 
the  membranes,  and  the  manometers  during  the  intervals  between  meas- 
urements. If  the  measurements  reported  in  the  present  chapter  were 
arranged  in  a  strictly  chronological  order,  it  would  be  observed  that  a  cell, 
once  used,  reappears  only  after  a  long  interval. 

It  was  intended,  in  the  beginning,  to  carry  the  measurement  of  the 
osmotic  pressure  of  cane  sugar  from  0°  to  100°,  or  as  near  to  the  latter 
temperature  as  possible.  The  temperature-intervals  selected  were  5° 
between  0°  and  30°,  and  10°  between  30°  and  100°.  The  work  progressed 
steadily  until  the  temperature  of  80°  was  reached,  when  it  was  necessary 
to  discontinue  the  measurements  for  the  three  summer  months.  The 
cells  were  allowed  to  cool  down  to  the  temperature  of  the  air  and  were 
then  placed  in  thymol  water  to  soak  through  the  summer.  No  serious 


164       OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

consequences  to  the  cells  were  apprehended  from  this  treatment;  for  in 
all  previous  work  at  moderate  and  low  temperatures  it  had  been  found 
that — provided  adequate  measures  were  taken  to  prevent  infection — the 
membranes  were  greatly  improved  by  the  customary  summer  soaking  in 
water  free  from  electrolytes.  We  were,  therefore,  wholly  unprepared  for 
the  calamity  which  resulted  (apparently)  from  too  rapid  cooling  of  the 
membranes  from  80°  to  the  temperature  of  the  air.  On  resuming  work  in 
the  fall,  it  was  found  that  none  of  the  membranes  would  sustain  the  full 
osmotic  pressures  of  the  solutions  either  at  high  or  moderate  tempera- 
tures. More  than  three  months  were  spent  in  applying  all  the  known 
means  for  the  restoration  and  improvement  of  membranes,  but  to  no 
purpose.  None  of  them  could  be  made  to  measure  pressure  at  any  tem- 
perature. It  was  evident  that  the  material  of  the  membranes  had  under- 
gone some  change  in  structure  which  robbed  them,  at  least  to  a  great 
extent,  of  their  semipermeable  character.  Moreover,  it  was  to  be  inferred 
from  the  impossibility  of  building  up  good  new  membranes  in  the  presence 
of  the  old  ones,  that  probably  a  similar  transformation  was  quickly  induced 
in  all  newly  deposited  membrane  material. 

Having  found  that  cells  which  had  formerly  been  in  excellent  condition 
for  work  at  high  temperatures  could  not  be  restored  to  a  usable  condition, 
they  were  consigned  to  a  solution  of  thymol  in  order  to  test  the  effect  of 
prolonged  soaking  in  water.  It  was  then  necessary  to  begin  again  at 
the  bottom,  that  is,  to  make  new  cells,  to  build  up  in  them  membranes  at 
some  moderate  temperature,  and  afterward  to  perfect  these  membranes  at 
higher  and  higher  temperature-intervals.  The  preparation  of  the  clays, 
and  the  making,  burning,  and  glazing  of  the  cells  require  considerable 
time,  but  by  no  means  as  much  as  the  "training"  of  the  membranes  for 
work  at  high  temperatures.  For  that  purpose  it  is  necessary  to  deposit 
the  first  membranes  at  a  low  or  moderate  temprature,  probably  not  above 
30°,  and  then  to  develop  them  at  that  temperature  until  they  are  found 
to  measure  osmotic  pressure  satisfactorily.  Though  measuring  perfectly 
at  30°,  they  will  be  found  defective  at  40°,  and  must  be  again  developed 
at  the  latter  temperature,  etc.  The  cells  with  which  the  work  reported 
in  this  chapter  is  to  be  resumed  at  70°  are  now  (15  months  after  begin- 
ning their  manufacture)  measuring  satisfactorily  at  50°. 

In  the  following  statement  of  the  results  obtained  between  0°  and 
80°,  the  few  data  which  accompany  each  of  the  270  records  of  observed 
pressures,  namely,  the  cell  used,  the  resistance  of  the  membrane,  and  the 
"initial"  pressure,  are  included  because  they  serve  to  illustrate  many  of 
the  points  which  have  been  made  in  previous  chapters.  After  these,  are 
given  the  mean  daily  pressures,  beginning  with  the  day  on  which  the  pres- 
sure was  supposed  to  have  reached  a  close  approximation  to  equilibrium. 
Several  readings  were  made  each  day,  and  it  is  the  mean  of  all  of  these 
which  is  to  be  understood  by  the  term  "mean  daily  pressure."  Between 
0°  and  25°  it  was  customary  to  correct  the  mean  of  the  total  pressures  of 


CANE    SUGAR. 


165 


each  day  by  the  mean  barometric  pressure  for  the  given  24  hours.  But 
between  30°  and  80°  the  mean  daily  total  pressures  are  recorded,  and  the 
correction  for  atmospheric  pressure  is  applied  by  deducting,  from  the  mean 
of  all  the  mean  daily  total  pressures,  the  mean  barometric  pressure  of  the 
whole  time  the  cells  were  under  observation.  This  change  in  practice 
was  due  to  the  fact  that,  as  the  membranes  grow  older,  the  duration  of 
barometer  effects,  as  well  as  of  thermometer  effects,  increases.  No  con- 
fusion will  result  from  the  change  in  the  form  of  stating  results,  if  it  is 
remembered  that  "total"  pressure  means  osmotic  plus  atmospheric  pres- 
sure, while  "osmotic"  pressure  means  total  observed  minus  atmospheric 
pressure. 

TABLE  47. — Determinations  of  osmotic  pressure  at  0°. 

(Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer,  and  E.  G.  Zies.) 

[W.  N.  S.  =  Weight  normal  solution.     C.  G.  P.  =  Calculated  gas  pressure.] 


Experiment  No. 

"3 
O 

Resistance  mem- 
brane. 

Manometer. 

Initial  pressuie. 

Observed  mean  daily  osmotic  pressure. 

Mean  osmotic  pres- 
sure for  experi- 
ments. 

Ratio  of  osmotic  to 
gas  pressure. 

>> 

•d 

-a 
a 

§ 

CK 

£ 
-o 

I 

3 
H 

>> 

S3 
T3 

•S 

It 

1 

>> 

c3 
T3 
^ 

iS 
E 

a 

M4 

M 

w    3 

Q     W 

i! 

% 

0.1  W.  N.  S.  at  0° 
C.G.  P.  2.227... 

0.2  W.  N.  S.  at  0° 
C.G.  P.  4.453... 

0.3  W.  N.  S.  at  0° 
C.G.  P.  6.680... 

0.4  W.  N.  S.  at  0° 
C.G.  P.  8.906... 
0.5  W.  N.  S.  at  0° 
C.G.  P.  11.133.. 
0.6  W.  N.  S.  at  0° 
C.G.  P.  13.359.. 
0.7  W.  N.  S.  at  0° 
C.  G.  P  15.586.. 
0.8  W.  N.  S.  at  0° 
C.G.  P.  17.812.. 
0.9  W.  N.  S.  at  0° 
C.G.  P.  20.04... 
1.0  W.  N.  S.  at  0° 
C.G.  P.  22.265.. 

& 

la 

J 

' 

I'- 
ll 
?i 

\l 

12 

ft 

U 

[1 

H 

i2 
fi 

12 
fl 
I* 

K, 
K3 
K3 
Z3 

Q2 
K3 
M3 
M3 
M3 
M3 
D3 
E3 
M3 
E3 
E3 
Q3 
M3 
D3 
H3 
D3 
F3 
D3 
D3 
D3 

545,000 
224,000 
226,000 
290,000 
220,000 
193,000 
278,000 
224,000 
224,000 
185,000 
236,000 
183,000 
160,000 
140,000 
151,000 
212,000 
515,000 
280,000 
366,000 
550,000 
550,000 
160,000 
180,000 
555,000 

13 
6 
6 
5 
6 
6 
6 
11 
11 
6 
13 
6 
11 
6 
6 
11 
5 
6 
5 
11 
11 
5 
20 
11 

2.20 
2.00 
2.30 
4.46 
4.70 
4.50 
4.60 
6.75 
7.0 
7.0 
9.51 
9.10 
11.80 
11.70 
14.02 
14.389 
16.82 
16.59 
19.10 
19.01 
21.45 
21.78 
24.35 
24.63 

2.461 
2.465 
2.463 
4.719 
4.731 
4.715 
4.726 
7.083 
7.113 
7.068 
9.440 
9.425 
11.914 
11.866 
14.364 
14.389 
16.888 
16.902 
19.485 
19.448 
22.163 
22.077 
24.883 
24.762 

2.456 
2.460 
2.460 
4.719 
4.727 
4.720 
4.727 
7.074 
7.106 
7.074 
9.460 
9.434 
11.912 
11.884 
14.370 
14.402 
16.875 
16.892 
19.496 
19.495 
22.135 
22.106 
24.878 
24.798 

2.460 
2.464 
2.463 
4.719 
4.730 
4.717 
4.726 
7.078 
7.107 
7.071 
9.450 
9.435 
11.907 
11.882 
14.367 
14.395 
16.881 
16.891 
19.486 
19.466 
22.149 
22.087 
24  .  878 
24.774 

[  2.462 
>  4.722 

[  7.085 

^  9.442 

^11.895 
J14.381 
|  16.  886 
J  19.  476 
J22.118 
J24.825 

1.106 
1.061 

1.061 

1.060 
1.068 
1.0765 
1.083 
1.093 
1.104 
1.115 

7.102 

9.440 
11.890 
11.912 

16.876 
19.478 
19.470 

19.452 

22.086 
24.864 
24.776 

It  will  be  seen  that  the  equilibrium  pressure  was  reached  in  many  cases 
on  the  second  day;  in  others,  on  the  third  day;  and  in  some,  only  after  sev- 
eral days.  This  apparent  inconsistency  has  been  explained  in  a  former 
chapter  as  due,  principally,  to  the  varying  ages  of  the  membranes;  it  hav- 
ing been  observed  that,  as  a  membrane  grows  older,  the  solvent  passes 
through  it  more  slowly.  There  are  other  minor  causes  of  differences  in  the 


166 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


activity  of  membranes,  but  they  need  not  be  discussed  in  the  present  con- 
nection. In  many  cases,  the  record  could  have  been  begun  earlier  than  it 
was,  but  there  is  need  of  caution  in  the  measurement  of  pressure  with 
"slow"  cells,  because  of  the  persistence  of  thermometer  effects  in  them. 
There  is  always  some  danger,  when  using  slow  cells,  that  a  thermometer 
effect  may  be  mistaken  for  an  equilibrium  pressure. 

TABLE  48. — Determinations  of  osmotic  pressure  at  5°. 

(Measurements  by  H.  N.  Morse,  W.  W.  Holland,  and  E.  E.  Gill.) 

[W.  N.  S.  =  Weight  normal  solution.     C.  G.  P.  =  Calculated  gas  pressure.] 


Experiment  No. 

13 
O 

Resistance  mem- 
brane. 

Manometer. 

Initial  pressure. 

Observed  mean  daily  osmotic  pressure. 

Mean  osmotic  pres- 
sure for  experi- 
ments. 

Ratio  of  osmotic  to 
gas  pressure. 

i 

T3 

*3 

£ 

E 

* 

T3 
T3 

a 

5? 

TJ 
"O 

H 

'£ 

H 

S? 

•0 
g 
hi 

1 

£ 

TJ 

M 

1 

o 

§£ 

8  i 

§  £ 

s  — 

a 

0.1  W.N.  S.at5° 
C.  G.  P.  2.267.. 

0.2W.N.S.at5° 
C.G.S.  4.535.  .. 
0.3  W.  N.  S.  at  5°. 
C.G.P.6.802.. 

0.4W.N.S.at5° 
C.G.P.9.07... 

0.5W.N.  S.at5° 
C.G.P.  11.34.. 
0.6  W.  N.  S.  at  5° 
C.G.P.  13.604. 
0.7  W.  N.  S.  at  5° 
C.G.P.  15.872. 
0.8  W.  N.  S.  at  5° 
C.G.P.  18.139. 
0.9W.N.S.  at  5° 
C.G.P.  20.406. 

LOW.  N.S.atS0 
C.G.P.  22.67.. 

P 
la 

{i 
{. 

P 

3 

6 

[1 
[\ 

{2 

{i 

2 
3 
4 
5 

K, 
L3 
J3 
F8 
J3 
J3 
K3 
F, 
E3 
K3 
H3 
E3 
J3 
L3 
M3 
J3 
J3 
B3 
B3 
Q3 
F3 
S, 
K3 
J3 
G3 

366,000 
550,000 
228,000 
565,000 
270,000 
366,000 
224,000 
550,000 
550,000 
550,000 
500,000 
550,000 
360,000 
370,000 
275,000 
270,000 
275,000 
550.000 
500,000 
550,000 
1,000,000 
550,000 
1,100,000 
1,000,000 
550.000 

13 

6 
13 
6 
9 
6 
9 
13 
22 
9 
9 
13 
11 
20 
9 
6 
20 
9 
9 
20 
21 
20 
9 
21 
.22 

2.30 
2.39 
2.36 
4.55 
4.70 
5.66 
6.89 
7.27 
8.79 
9.28 
12.00 
10.74 
10.28 
13.59 
15.51 
16.00 
19.50 
16.61 
17.11 
22.06 
18.56 
20.12 
21.83 
20.15 
20.91 

2.455 
2.453 
2.454 

2.449 
2.449 
2.452 
4.815 

2.452 
2.451 
2.453 
4.812 
4.825 
7.187 
7.209 
9.623 
9.584 
9.617 
12.10 
12.10 
14.604 
14.606 
17.217 
17.194 
19.795 
19.849 
22.51 
22.443 
25.30 
25.30 
25.30 
25.24 
25.26 

[  2.452 

}  4.818 
}  7.198 

[  9.608 

J12.10 
|  14.  605 
J17.206 

J19.822 
J22.478 

25.28 

1.082 

1.063 
1.058 

1.059 

1.067 
1.074 
1.084 
1.093 
1.102 

1.115 

4.810 

4.838 
7.189 

4.822 
7.186 
7.213 

7.198 

9!  591 
9.644 
12.10 
12.11 
14.611 
14.605 
17.228 
17.191 
19  .  797 
19.849 

9.622 
9.577 
9.631 
12.10 
12.09 
14.597 
14.608 
17.207 
17.198 
19.795 
19.850 
22.46 
22.448 

9.624 

9.517 
12.09 

19.793 

19.793 

22.53 
22  .  439 

22.53 

25.32 
25.23 

25.31 

25.31 
25.25 
25.26 

25.29 
25.29 
25.26 
25.24 
25.25 

25.30 
25.31 

The  length  of  the  record  which  the  cells  were  allowed  to  make,  after 
having  reached  equilibrium  pressure,  varies  in  general  from  2  to  15  days. 
In  one  case — that  of  the  0.5  weight-normal  solution  at  15° — the  record 
was  prolonged  to  60  days,  in  order  to  test  the  endurance  of  the  membrane. 
As  a  rule,  however,  the  length  of  time  a  cell  was  allowed  to  continue  its 
record  depended  principally  upon  the  activity  of  the  membrane.  If  the 
cell  was  slow  in  coming  to  equilibrium,  it  was  allowed  to  remain  longer  hi 
the  bath,  in  order  to  lessen  the  errors  due  to  thermometer  and  barometer 
effects.  Sometimes  a  cell  was  allowed  to  continue  its  record  simply 
because  its  manometer  was  not  needed  for  another  cell. 


CANE    SUGAR. 


167 


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168 


OSMOTIC   PRESSURE   OF   AQUEOUS   SOLUTIONS. 


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CANE    SUGAR. 


169 


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170 


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CANE    SUGAR. 


171 


TABLE  51.  —  Determinations  of  osmotic  pressure  at  SO0  —  Continued. 

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172 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


TABLE  52.  —  Determinations  of  osmotic  pressure  at  25°. 
Measurements  by  H.  W.  Morse,  W.  W.  Holland,  and  C.  N.  Myers.  W.  N.  S.  =  Weight  normal  solution.  C.  G.  P.  =  Calculated  gas  pressure. 

Observed  mean  daily  osmotic  pressure. 

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CANE   SUGAR. 


173 


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174 


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CANE   SUGAR. 


175 


TABLE  53.  —  Determinations  of  osmotic  pressure  at  SO0.  —  Continued.* 

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*Beginning  with  30°,  the  correction  for  atmospheric  pressure  is  not  applied  each  day,  but  at  the  end  of  the  experiment,  when  the  mean  barometric  pressure 
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176 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


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^n         oSO 


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0 


**.  **        ** 


O 


ti 


178 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


-2 

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CANE   SUGAR. 


179 


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180 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


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CANE   SUGAR. 


181 


1 


182 


OSMOTIC   PRESSURE   OF  AQUEOUS  SOLUTIONS. 


TABLE  57. — Determinations  of  osmotic  pressure  at  70°. 

Measurements  by  H.  N.  Morse,  W.  W.  Holland,  and  J.  B  Zinn.    W.  N.  S.  =  Weight  normal  solution. 

C.  G.  P.  =  Calculated  gas  pressure. 


& 

0,-tf 

*l 

jjj 

O 

Resistance 
mem- 
brane. 

Manom- 

•"?  o> 

•21 

1  'SI 

M 

Observed  mean  daily  total  pressures. 

2d 
day. 

3d 
day. 

4th 
day. 

5th 
day. 

6th 
day. 

0.5  W.  N.  S.  at  70°  C.  G.  P. 
13.992  

{I 
{I 
{I 
[1 

• 

I     3 

i 

11 

Es 
Lt 
B6 
W8 
Js 
F6 
Us 
Rs 
Fs 
R5 

c» 

Gt 
LB 
Es 
R* 

13,000 
9,600 
10,000 
16,000 
8,000 
10,000 
23,000 
10,000 
19,000 
16,500 
7,700 
30,000 
33,000 
20,000 
33,000 

i 

38 
31 
15 
38 
24 
31 
31 
24 
28 
1 
28 
5 
1 
15 

,    17.70 
15.84 
17.81 
17.69 
19.93 
21.42 
25.58 
25.52 
28.71 
30.70 
o    25.79 
34.73 
31.12 
D    32.57 
31.87 

15.015 
14.984 
17.824 

14.986 
14.981 
17.816 
17.859 

14.985 
14.979 
17.800 
17.822 

14.993 
17.820 

0.6  W.  N.  S.  at  70°  C.  G.  P. 
16.790  

17.812 

0.7  W.  N.  S.  at  70°  C.  G.  P. 
19.589  

20.559 

20.574 

20.600 

20.592 

0.8  W.  N.  S.  at  70°  C.  G.  P. 
22.387  

23.568 

23.565 

23.556 

23.583 

0.9  W.  N.  S.  at  70°  C.  G.  P. 

26.589 
26.534 
29.542 
29.720 
29.602 
29.628 

26.589 
26.552 
29.558 
29.746 
29.602 
29.635 

1.0  W.  N.  S.  at  70°  C.  G.  P. 
27.984  

26.552 

29.676 

29.623 
29.572 
29.608 

Observed  mean  daily  total  pressures. 

7th 
day. 

8th 
day. 

9th 
day. 

10th 
day. 

llth 
day. 

12th 
day. 

13th 
day. 

14th 
day. 

15th 
day. 

0.5  W.  N.  S.  at  70°  C.  G. 
P.  13.992  

(M 
IM 

J17 
\17 
/2C 
\2C 

m 

12S 

.978 
.981 
.795 
.817 
.657 
.678 
.555 
.576 

14.974 
14.985 

0.6  W.  N.  S.  at  70°  C.  G. 
P.  16.790  

17.801 
20.564 
20.566 
23.549 

17.817 
20.516 

17.800 
20.516 

0.7  W.  N.  S.  at  70°  C.  G. 
P.  19.589  

20.528 

20.534 

0.8  W.  N.  S.  at  70°  C.  G. 
P.  22.387  

23.547 

23.562 

23.574 

23.668 

0.9  W.  N.  S.  at  70°  C.  G. 

26.543 

26.593 

26.556 

1  26.  565 

26.559 
26.563 
29.630 
29.715 
29.617 
29.628 

26.551 
26.541 
29.591 
29.635 
29.610 
29.600 

P.  25.186  

1.0  W.  N.  S.  at  70°  C.  G. 
P.  27.984  

[26.564 
[29.559 
1  29.  720 
129.602 
[29.621 

29.585 
29.618 
29.690 
29.590 

29.589 
29.617 
29.587 
29.596 

29.607 
29.646 
29.646 
29.647 

29.610 
29.629 
29.632 
29.700 

Observed  mean  daily  total  pressures. 

*,  ft 

0 

2  si 

™  *3  IN 

0    C3    3 
jg-O. 

&j 

I-     U 

ca  a 

•°B 

si* 

w  a  a 
jgS« 

It 

O     M 

flS 

gft 

2 

O    ki      . 

'•g-2  2 
i.  I 

•    M    8 

111 
IKI 

s    • 

A  3 

o  g 

o2j2 

o  '-3  TO 

s» 

16th 
day. 

17th 
day. 

18th 
day. 

19th 
day. 

0.5  W.  N.  S.  at  70°  C.  G. 
P   13  992 

r 

14.988 
14.984 
17.810 
17.819 
20.543 
20.577 
23.559 
23.570 
26.557 
26.571 
26.551 
29.590 
29.668 
29.606 
29.625 

0.994 
0.994 
0.995 
0.995 
0.990 
0.994 
0.992 
1.003 
0.998 
0.997 
0.997 
1.000 
0.997 
0.999X 
0.999 

13.994   1 
13.990 
16.815  ' 
16.824   , 
19.553   ' 
19.583 
22.567 
22.567 
25.559 
25.574 
25.554 
28.590  ' 
28.671 
28.607 
28.626 

•13.9905 
16.8195 
•19.568 
22.567 

25.562 
'28.6235 

1.000 
1.0018 
0.999 
1.008 

1.015 
1.0228 

\ 

0.6  W.  N.  S.  at  70°  C.  G. 
P.  16.790  

f 

{ 

0.7  W.  N.  S.  at  70°  C.  G. 
P.  19.589       

\ 

( 

0.8  W.  N.  S.  at  70°  C.  G. 
P.  22.387     

| 

0.9  W.  N.  S.  at  70°  C.  G. 
P.  25.186  

f26 

562 

26.523 

26.549 

26.574 



1.0W.N.S.at70°C.G. 
P.  27.984  

f::::::: 

; 

. 

CANE   SUGAR. 

TABLE  58. — Determinations  of  osmotic  pressure  at  80°. 
W.  N.  S.  =  Weight  normal  solution.     C.  G.  P.  =  Calculated  gas  pressure. 


183 


6 

'a 

V 

1 

=3 
o 

Resistance  mem- 
brane. 

Manometer. 

Initial  pressure. 

Observed  mean  daily  total  pressures. 

Second 
day. 

Third 
day. 

Fourth 
day. 

Fifth 
day. 

Sixth 
day. 

Seventh 
day. 

0.8  W.  N.  S.  at  80° 
C.  G.  P.  23.041  .  . 
0.9  W.  N.  S.  at  80° 
C.  G.  P.  25.921  .  . 
1.0  W.  N.  S.  at  80° 
C.  G.  P.  28.801  .  . 

to  h->  tO  I-1  )-« 

rwpww 

9,000 
10,000 
10,000 
10,500 
10,000 

24 
31 

31 

lo 

33.00 
26.85 
23.96 
30.20 
28.19 

24.087 
26.909 
26.933 
29.780 
29.770 

24.073 
26.914 
26.909 

24.042 
26.919 
26.873 

26.905 
26.974 
29.801 

29.838 

29.811 

29.863 

29.836 

Observed  mean  daily  total  pressures. 

'3 

~3  £ 

•x  3 
o  <u 

P 

03 

Mean  barometric 
pressure. 

Mean  osmotic  pres- 
sure. 

Mean  osmotic  pres- 
sure for  experi- 
ments. 

Ratio  of  osmotic  to 
gas  pressure. 

•o 

1 

S 

•o 

.a 
fc 

S? 
•o 

3 

a 

« 

H 

Eleventh  day. 

Twelfth  day. 

0.8  W.  N.  S.  at  80° 
C.  G.  P.  23.041.. 

24  036 

24.060 
26.912 
26.920 
29.808 
29.8175 

0.998 
0.997 
0.998 
0.992 
0.998 

23.062 
25.915 
25.922 
28.816 
28.8195 

1.001 
1.000 

1.000 

0.9  W.  N.  S.  at  80° 
C.G.  P.  25.921.. 
l.OW.N.S.atSO0 
C.  G.  P.  28.801  .  .  * 

r.. 

J25.919 

J28.8178 

[26.909 

r 

[29.799 

29.785 

29.829 

29.859 

29.799 

The  results  of  the  foregoing  determinations  of  osmotic  pressure  have 
been  brought  together  in  tables  59  to  63.  Table  59  gives,  for  each  con- 
centration of  solution  and  each  temperature,  the  observed  osmotic 
pressure.  Table  60  gives  the  means  of  the  observed  osmotic  pressures 
for  each  concentration  and  temperature.  Table  61  contains  the  calcu- 
lated gas  pressure  of  the  solute  for  all  concentrations  of  solution  which 
were  employed,  and  for  all  the  temperatures  at  which  measurements  of 
osmotic  pressure  were  made — the  volume  of  the  gas  being  that  of  the  solvent 
in  the  pure  state,  and  not  that  of  the  solution.  Table  62  gives  the  ratios 
of  the  observed  osmotic  pressures  of  the  solutions  to  the  calculated  gas 
pressures  of  the  solute,  i.  e.,  the  results  which  are  obtained  by  divid- 
ing the  values  in  Table  60  by  the  corresponding  values  in  Table  61. 
In  order  to  facilitate  an  interpretation  of  the  results,  Tables  60  and  62 
have  been  divided  into  three  sections  by  means  of  heavy  lines.  Attention 
is  called  more  especially  to  Table  62,  in  which  are  given  the  ratios  of 
osmotic  to  gas  pressure.  If  the  values  on  the  various  horizontal  lines, 
to  the  left  of  the  vertical  heavy  line,  are  compared,  it  will  be  seen  that  the 
ratio  of  osmotic  to  gas  pressure,  between  0°  and  25°,  is  very  nearly  con- 
stant for  each  concentration  of  solution.  Omitting  the  ratio  for  the  0.1 
weight-normal  solutionat  0°,  the  means  of  the  ratios  for  the  various  con- 
centrations are  given  in  Table  63. 


184 


OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


TABLE  59. — Cane  sugar.    Osmotic  pressures  between  0°  and  80°. 


Con- 
centra- 
tion. 

0°. 

5°. 

10°. 

15°. 

20°. 

25°. 

30°. 

40°. 

50°. 

60°. 

70°. 

80°. 

0.1 
0.2 

0.3 
0.4 
0.5 
0.6 

2.460 
2.464 
2.463 

2.452 
2.451 
2.453 

2.494 
2.496 
2.502 
2.498 

2.547 
2.535 
2.541 
2.538 

2.589 
2.590 

2.635 
2.632 

2.476 
2.472 
2.468 
2  480 

2.555 
2.562 
2.563 

2.643 
2.638 
2.629 

2.720 
2.714 

4.719 
4.717 
4.730 
4.726 

4.812 
4.825 

4.890 
4.896 

4.981 
4.988 

5.058 
5.056 
5.066 

5.154 
5.139 
5.150 

5.040 
5.048 

5.168 
5.158 

5.273 
5.286 
5.277 

5.450 
5.425 

5.065 

5.074 

7.078 
7.107 
7.071 

7.187 
7.209 

7.332 
7.337 

7.465 
7.486 

7.586 
7.624 
7.606 

7.735 
7.719 
7.722 

7.641 
7.653 

7.817 
7.853 
7.831 

7.960 

7.988 
7.974 

8.152 
8.128 

7.738 

7.835 

• 

9.450 
9.435 

9.623 
9.584 
9.617 

9.791 
9.790 

9.950 
9.947 

10.136 
10.138 

10.301 
10.295 

10.305 
10.285 

10.599 
10.603 
10.607 

10.737 
10.710 

10.871 
10.853 
10.873 

10.586 

11.907 
11.882 

12.100 
12.100 

12.296 
12.298 

12.565 
12.533 

12.742 
12.754 

12.932 
12.972 
12.919 

12.980 
12.975 

13.349 
13.361 

13.515 
13.493 
13  .  504 

13.645 
13.687 

13.994 
13.990 

12.947 

14.367 

14.604 

14.856 

15.128 

15.405 

15.632 

15.706 

16.137 

16.322 

16.519 

16.815 

0.7 

14.395 

14.606 

14  .  854 

15.16015.370 

15.615 

15.694 

16.152 

16.306 

16.551 

16  .  824 

16.881 

17.217 

17.488 

:::::: 

17.821 

18.135 

15.62015.763 
15.63415.690 
18.43618.494 

18.962 

19.225 

19.403 

19.553 

0.8 

0.9 
1.0 

16.891 
19.486 
19.466 

17.194 
19.795 
19.849 

17.51817.80818.121 
20.15220.53320.928 
20.169[20.52520.883 

18.434 
21.250 
21  .  258 

18.505 
21.396 
21.354 

18.901 
21.779 
21.813 

19.179 
22.129 
22  .  104 

19.405 
22.344 
22.310 

19.583 
22.567 
22.567 

23.062 

20.548 

20.899 
20.909 

21.819 
21.811 

22.149 
22.087 

22.443 
22.510 

22.911 
22.857 

23.314 
23.296 

23.715 
23.718 

24.126 
24.125 

24.215 
24.238 

24.738 
24.730 
24  .  738 

25.132 
25.114 

25.256 
25.275 

25.559 
25.574 
25  .  554 

25.915 
25.922 

24.878 
24.774 

25.300 
25.300 
25.300 
25.240 
25  .  260 

25.704 
25.682 

26.206 
26.171 

26.648 
26.627 

27.030 
27.076 

27.240 
27.215 
27.242 
27.196 

27.673 
27.743 

27.688 

28.199 
28.231 
28.196 

28.357 
28.376 

28.590 
28.671 
28.607 
28.626 

28.816 
28.820 

TABLE  60. — Cane  sugar.     Mean  osmotic  pressures  between  0°  and  80°. 


Con- 
centra- 
tion. 

0°. 

5°. 

10°. 

15°. 

20°. 

25°. 

30°. 

40°. 

50°. 

60°. 

70°. 

80°. 

0.1 
0.2 
0.3 
0.4 
0.5 
0.6 
0.7 
0.8 
0.9 
1.0 

(2.462) 
4.723 
7.085 
9.443 
11.895 
14.381 
16.886 
19.476 
22.118 
24.826 

2.452 
4.819 
7.198 
9.608 
12.100 
14.605 
17.206 
19.822 
22.477 
25.280 

2.498 
4.893 
7.335 
9.790 
12.297 
14.855 
17.503 
20.161 
22.884 
25.693 

2.540 
4.985 
7.476 
9.949 
12.549 
15.144 
17.815 
20.535 
23.305 
26.189 

2.590 
5.064 
7.605 
10.137 
12.748 
15.388 
18.128 
20.905 
23.717 
26.638 

2.634 
5.148 
7.729 
10.296 
12.943 
15.625 
18.435 
21.254 
24.126 
27.053 

2.474 

2.560 

2.637 
5.279 

2.717 
5.438 
8.140 
10.866 

5.044 
7.647 
10.295 
12.978 
15.713 
18.499 
21.375 
24.226 
27.223 

5.163 
7.834 
10.599 
13.355 
16.146 
18.932 
21.806 
24.735 
27.701 

7.974 
10.724 
13.504 
16.314 
19.202 
22.116 
25.123 
28.209 

13.666 
16.535 
19.404 
22.327 
25.266 
28.367 

13.991 
16.820 
19.568 

22.567 
25.562 
28.624 

23.062 
25.919 

28.818 

CANE   SUGAR. 


185 


The  average  deviation  from  these  mean  ratios  is  0.15  per  cent,  while 
the  largest  single  deviation — that  of  the  0.6  normal  solution  at  25° — is 
0.3  per  cent.  It  is  obvious  from  the  relations  pointed  out  above,  that 
between  0°  and  25°  the  osmotic  pressure  of  cane-sugar  solutions — rang- 
ing in  concentration  from  0.1  to  1.0  weight-normal — obeys  the  law  of 
Gay-Lussac  for  gases.  In  other  words,  within  the  limits  designated, 
the  temperature  coefficients  of  gas  and  osmotic  pressures  are  identical. 
So  much  must  be  conceded  on  the  basis  of  the  experimentally  demon- 
strated facts — whatever  may  hereafter  be  found  to  be  true  of  solutions  of 
cane  sugar  which  are  more  or  less  concentrated,  or  of  other  substances. 

TABLE  61. — Cane  sugar.    Calculated  gas  pressures  of  solute  between  0°  and  80°. 


Cone. 

0°. 

5°. 

10°. 

15°. 

20°. 

25°. 

30°. 

40°. 

50°. 

60°. 

70°. 

80°. 

0.1 

2.227 

2.267 

2.308 

2.349 

2.390 

2.431 

2.472 

2.553 

2.635 

2.717 

2.798 

2.880 

0.2 

4.453 

4.535 

4.616 

4.698 

4.780 

4.862 

4.943 

5.107 

5.270 

5.433 

5.597 

5.760 

0.3 

6.680 

6.802 

6.925 

7.047 

7.170 

7.292 

7.415 

7.660 

7.905 

8.150 

8.395 

8.640 

0.4 

8.906 

9.069 

9.233 

9.396 

9.560 

9.723 

9.88610.213 

10.540 

10.867 

11.194 

11.520 

0.5 

11.133 

11.337 

11.541 

11.745 

11.950 

12.15412.358 

12.767 

13.175 

13.584 

13.992 

14.401 

0.6 

13.359 

13.604 

13.849 

14.094 

14.339 

14  .  585  14  .  830  15  .  320  15  .  810 

16.300 

16.790 

17.281 

0.7 

15.585 

15.871 

16.157 

16.443 

16.729 

17.015 

17.301 

17.873 

18.445 

19.017 

19.589 

20.161 

0.8 

17.812 

18.139 

18.466 

18.792 

19.119 

19.446 

19.773 

20.426 

21.080 

21.734 

22.387:23.041 

0.9 

20.038 

20.406 

20.774 

21.141 

21.509 

21.877 

22.24422.980 

23.71524.45025.18625.921 

1.0 

22.26522.674 

23.082 

23.490 

23.899 

24.30824.71625.533 

26.35027.167 

27.984 

28.801 

TABLE  63. 


The  0.1  normal  solution  at  0°  appears  to  present  an  exception  to  the 
rule  that  the  ratio  of  osmotic  to  gas  pressure  for  that  concentration  is 
about  1.083  at  temperatures  under  25°.  The  pressure  found  was  2.462, 
which  gives  a  ratio  to  calculated  gas  pressure  of  1.106; 
whereas,  in  order  to  conform  to  the  rule  which  holds 
for  the  0.1  normal  solution  at  5°,  10°,  15°,  20°,  and 
25°,  the  pressure  should  be  about  2.227x1.083  equals 
2.413.  The  difference,  2.462  minus  2.413  equals  0.049 
atmosphere,  is  probably  too  large  to  be  accounted  for 
as  due  to  experimental  error — especially  in  a  solution 
so  dilute  that  unavoidable  errors  of  meniscus  and 
capillary  depression  are  of  little  moment.  More- 
over, thermometer  effects  which  are  a  source  of 
sensible  error  at  other  temperatures  are  insignificant 
in  a  properly  constructed  bath  at  0°.  It  is  to  be  remembered  in  this 
connection,  as  possibly  explaining  the  apparent  anomaly,  that  at  0°  the 
0. 1  normal  solution  is  within  less  than  0.2°  of  its  freezing  temp  erature.  It 
is  desirable  to  investigate  more  concentrated  solutions  at  temperatures 
equally  near  their  freezing-points,  but  such  investigations  will  obviously 
be  attended  by  great,  if  not  insurmountable,  experimental  difficulties. 
The  problem  of  maintaining  constant  temperatures  below  0°  is  in  itself 
a  difficult  one.  Moreover,  at  temperatures  below  the  freezing-point  of 
the  solvent,  the  osmotic  pressures  of  solutions  can  be  measured  only 
by  differential  methods;  that  is,  the  cells  must  be  surrounded  by  solu- 


Concen- 
tration. 

Mean 
ratio. 

0.1 

1.083 

0.2 

1.061 

0.3 

1.060 

0.4 

1.060 

9.5 

1.067 

0.6 

1.074 

0.7 

1.083 

0.8 

1.093 

0.9 

1.103 

1.0 

1.114 

186 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


tions  only  a  little  less  dilute  than  those  whose  osmotic  pressure  is  to 
be  determined. 

The  ratios  of  osmotic  to  gas  pressure  between  0°  and  25°,  though  con- 
stant for  each  concentration,  are  all  greater  than  unity.  The  excess 
varies  from  6  per  cent  in  the  0.2,  0.3,  and  0.4  normal  solutions,  on  the 
one  side,  to  8.3  per  cent  in  the 0.1  normal  solution;  and  on  the  other,  to 
11.4  per  cent  in  the  normal  solution.  The  increase  in  ratio  from  the  0.4 
through  the  succeeding  concentrations  exhibits  a  certain  amount  of 
regularity.  The  increment  between  the  0.4  and  0.5,  and  also  between 
the  0.5  and  0.6,  is  about  0.7  per  cent.  All  the  succeeding  increments, 
i.  e.,  those  between  the  0.6  and  0.7,  the  0.7  and  0.8,  the  0.8  and  0.9,  and 
the  0.9  and  1 .0  concentrations,  are  approximately  1 .0  per  cent.  A  notice- 
able feature  of  the  0.2,  0.3,  and  0.4  weight-normal  solutions  is  the  fact 
that  their  osmotic  pressures  are  all  about  equally  (6  per  cent)  in  excess 
of  the  calculated  gas  pressure  of  the  solute. 

TABLE  62. — Cane  sugar.    Ratio  of  osmotic  to  gas  pressure. 


Cone. 

0°. 

5°. 

10°. 

15°. 

20°. 

25°. 

30°. 

40°. 

50°. 

60°. 

70°. 

80°. 

0.1 

(1.106) 

1.082 

1.082 

1.082 

1.084 

1.084 

1.000 

1.003 

1.000 

1.000 

0.2 

1.061 

1.063 

1.060 

1.061 

1.062 

1.059 

1.020 

1.011 

1.002 

1.001 

0  3 

1  061 

1.058 

1.059 

1.061 

1.060 

1.060 

1.031 

1.024 

1.009 

0.999 

0.4 

1.060 

1.059 

1.060 

1.059 

1.060 

1.059 

1.040 

1.038 

1.017 

1.000 

0  5 

1  068 

1.067 

1.066 

1.068 

1.067 

1.065 

1.050 

1.046 

1.025 

1.006 

1.000 

0  6 

1  .  0765 

1.074 

1.073 

1.073 

1.073 

1.071 

1.060 

1.054 

1.032 

1.014 

1.002 

0.7 

1.083 

1.084 

1.083 

1.083 

1.084 

1.083 

1.069 

1.059 

1.041 

1.020 

0.999 

0.8 
0.9 
1.0 

1.093 
1.104 
1.115 

1.093 
1.102 
1.115 

1.092 
1.102 
1.113 

1.093 
1.102 
1.115 

1.093 
1.103 
1.115 

1.093 
1.102 
1.113 

1.081 
1.089 
1.101 

1.067 
1.076 
1.085 

1.049 
1.059 
1.071 

1.027 
1.033 
1.044 

1.008 
1.015 
1.023 

1.001 
1.000 
1.000 

Having  found  that  the  law  of  Gay-Lussac  does  hold  for  the  osmotic 
pressures  of  cane-sugar  solutions  between  0°  and  25°,  one  is  inclined  to 
believe  that  they  should  also  conform  to  the  law  of  Boyle,  and  to  seek 
for  some  rational  explanation  of  the  facts:  1st,  that  the  ratios  in  ques- 
tion are  excessive,  i.  e.,  above  unity;  and  2d,  that  they  are  not  propor- 
tional to  the  supposed  concentration  of  the  solutions.  The  most 
obvious  general  explanation  (if  one  attempts  to  reconcile  the  pressures 
between  0°  and  25°  to  the  view  that  the  law  of  Boyle,  as  well  as  that  of 
Gay-Lussac,  does  hold)  is  hydration  of  the  solute,  which  may  be  pre- 
sumed to  have  the  effect  of  concentrating  the  solutions.  But  if  one 
attempts  to  work  out  the  precise  degrees  of  hydration  which  would 
account  for  the  variations  of  ratio  from  concentration  to  concentration, 
he  is  quickly  entangled  in  certain  hazardous  assumptions  respecting  the 
relations  of  solvent  to  solute  and  the  effect  of  these  upon  the  osmotic 
pressure.  In  the  writer's  opinion,  judgment  as  to  the  applicability  of 
Boyle's  law  to  the  osmotic  pressure  of  cane-sugar  solutions  at  tempera- 
tures below  25°  should  be  suspended  until  much  more  is  known  about 
the  osmotic  pressures  of  the  aqueous  solutions  of  other  substances. 


CANE    SUGAR.  187 

The  half  of  Table  62  which  lies  to  the  right  of  the  heavy  vertical  line  is 
divided  into  two  areas  by  a  heavy  zig-zag  line,  which  begins  at  the  top 
between  25°  and  30°,  and  ends  at  the  bottom  between  70°  and  80°.  All 
the  ratios  between  the  vertical  and  the  zig-zag  lines  are  greater  than 
unity,  but  decrease  continuously  with  rising  temperature.  The  ratios 
to  the  right  and  above  the  zig-zag  line  are  unity  within  necessary  experi- 
mental errors. 

The  situation  disclosed  in  Table  62  may  be  summed  up  as  follows: 
Between  0°  and  25°,  the  ratios  of  osmotic  to  gas  pressure  are  all  greater 
than  unity,  but  constant  for  each  concentration.  At  some  temper- 
ature between  25°  and  30°,  these  ratios  begin  to  decline,  but  relatively 
more  rapidly  in  the  dilute  than  in  the  concentrated  solutions.  At 
some  temperature  (30°  for  0.1;  50°  for  0.2;  60°  for  0.3  and  0.4;  70°  for 
0.5,  0.6,  and  0.7;  and  80°  for  0.8, 0.9,  and  1.0)  the  ratio  becomes  unity  for 
every  concentration. 

The  decrease  in  the  ratios  of  osmotic  to  gas  pressure  at  temperatures 
above  25°  suggests  an  increasing  dilution  of  the  solutions  through  the 
dissociation  of  unstable  hydrates;  and  it  serves  to  strengthen  the 
impression  that  the  excessive  but  constant  ratios  below  25°  are  due  to 
the  presence  of  stable  hydrates. 

However  the  excessive-constant  and  the  excessive-declining  ratios 
may  be  explained,  it  is  clear  that  at  30°,  40°,  50°,  and  60°  the  osmotic 
pressure  of  the  0.1  weight-normal  solution  obeys  both  of  the  gas  laws. 
The  same  may  safely  be  affirmed  of  the  0.2  normal  at  50°  and  60°;  of 
the  0.3  and  0.4  at  60°;  of  the  0.5, 0.6,  and  0.7  at  70°;  and  of  the  0.8,  0.9, 
and  1.0  at  80°.  It  is  now  important  to  ascertain  whether  the  ratios, 
having  once  declined  to  unity,  maintain  that  value  at  all  higher  tem- 
peratures; hence  the  work  of  measuring  the  osmotic  pressure  of  cane 
sugar  at  70°  and  80°,  and  at  still  higher  temperatures,  will  be  resumed 
as  soon  as  the  new  cells,  previously  referred  to,  have  been  sufficiently 
developed  for  use  at  those  temperatures.  The  development  of  mem- 
branes from  a  satisfactory  condition  at  30°  to  an  efficient  state  at  70° 
or  80°  will  probably  require  about  one  year. 

Special  attention  is  again  called  to  experiment  2  with  the  0.5  weight- 
normal  solution  at  15°,  where  the  full  osmotic  pressure  was  maintained 
by  the  cell  for  60  days  without  any  evidence,  at  the  end  of  that  time,  of 
a  weakening  of  the  membrane.  The  experiment  is  important,  not  only 
because  it  proves  the  membrane  to  have  great  endurance,  but  also 
because  of  the  light  which  it  throws  upon  the  question  of  the  deteriora- 
tion of  the  membranes  while  in  contact  with  a  solute.  The  cell  (C5) 
which  was  used  in  the  experiment  in  question  was  brought  again  into  a 
usable  condition  within  the  time  ordinarily  required  for  that  purpose, 
and  was  subsequently  employed  for  eight  more  determinations  which 
are  recorded  in  the  present  chapter.  A  membrane  which  had  been  a 
longtime  in  contact  with  a  0.4  weight-normal  solution  of  lithium  chloride 
required  for  its  restoration  more  than  20  months  of  soaking  in  water. 


CHAPTER  IX. 
GLUCOSE. 

FINAL  DETERMINATIONS  OF  OSMOTIC  PRESSURE. 

The  conditions  under  which  the  determinations  recorded  in  this 
chapter  were  made  were  the  same  as  for  the  " final"  measurements  of 
the  osmotic  pressure  of  cane  sugar.  There  was  no  sensible  change  in 
the  rotation  of  the  solutions  while  in  the  cells;  and  there  was,  therefore, 
no  gain  or  loss  in  their  concentration.  The  temperature  maintained 
in  the  baths  was  constant  to  within  0.02°,  except  when  some  unforeseen 
accident  happened  to  the  regulating  system.  It  has  already  been 
stated  that  the  usual  cause  of  such  accidents  is  a  temporary  failure  of 
the  current  at  the  power  house.  If  this  is  promptly  discovered,  serious 
results  may  be  avoided  by  switching  the  regulating  devices  to  the 
storage  battery.  The  normal  effect  of  a  break  in  the  current  is,  of 
course,  a  drop  in  the  temperature  of  the  baths.  Occasionally,  the 
temperature  of  the  baths  is  forced  up  above  that  for  which  the  thermo- 
stats are  set,  but  this  is  always  due  to  a  failure  to  make  the  "  cooling 
margin"  sufficiently  ample  to  cover  all  possible  fluctuations  in  external 
temperature  conditions.  In  practice  it  rarely  happens,  except  when 
measuring  osmotic  pressure  near  the  temperature  of  the  outside  air. 
Whenever  a  deviation  in  bath  temperature  has  been  sufficient  to  produce 
a  really  serious  thermometer  effect,  the  fact  has  been  made  apparent  in 
this  report  by  omitting  from  the  record  the  readings  of  the  day  or  days 
through  which  the  thermometer  effect  persisted. 

The  material  employed  for  the  determinations  was  the  same  as  that 
used  for  the  measurements  reported  in  Chapter  VII. 

It  was  intended  originally  to  begin  the  " final"  measurements  of  the 
osmotic  pressure  of  glucose  solutions  at  0°,  and  then,  as  in  the  case  of 
cane  sugar,  to  repeat  the  work  at  each  higher  interval  of  5°  or  10°  in 
regular  order.  But  the  disaster  to  the  cells  explained  previously  made  a 
change  in  plan  advisable.  It  was  desired  to  resume  and  complete  the 
work  on  cane  sugar  at  high  temperatures  with  the  least  possible  loss 
of  time;  and  to  this  end  the  deposition  and  development  of  the  mem- 
branes in  the  new  cells  were  begun  at  the  highest  practicable  temperature, 
namely,  30°.  But  as  the  new  cells,  after  serving  at  high  temperatures, 
might  also  be  lost  on  returning  to  ordinary  or  low  temperatures,  it  was 
decided  to  measure  the  osmotic  pressure  of  glucose  at  each  temperature- 
interval  for  which  the  membranes  must  be  developed  before  resuming 
the  work  upon  cane  sugar.  In  other  words,  it  was  decided,  after 

188 


GLUCOSE.  189 

having  developed  the  membranes  at  30°,  to  measure  the  pressures  of 
glucose  at  that  temperature  before  proceeding  to  develop  them  at  40°; 
and,  having  perfected  them  at  40°,  to  measure  again  the  pressures  of 
glucose  before  proceeding  to  50°,  etc.  On  reaching  70°  and  80°,  at 
which  the  measurement  of  the  pressures  of  cane  sugar  was  discontinued, 
it  is  intended  to  determine  the  osmotic  pressure  of  both  substances 
concurrently,  for  those  and  for  all  higher  temperatures.  It  has  been 
suggested  in  a  former  chapter  that  perhaps  membranes  which  have 
served  at  high  temperatures  may  be  saved  for  work  at  lower  tempera- 
tures by  reversing  the  process  by  which  they  were  developed,  that  is, 
by  perfecting  them  at  short  temperature-intervals  in  the  descending 
order.  This  will  be  attempted  after  finishing  the  work  upon  glucose 
and  cane  sugar  at  high  temperatures.  The  prospect  for  success  is  not 
regarded  as  very  good;  since,  hitherto,  it  has  been  found  quite  imprac- 
ticable to  rebuild  effective  membranes  out  of  old  ones  which  have 
largely  lost  their  semi-permeable  character.  It  appears  to  make  little 
difference  whether  the  damage  to  the  membranes  has  resulted  from  a 
great  and  too  rapid  fall  in  temperature,  or  from  the  action  of  electro- 
lytes upon  them.  The  only  remedy  for  loss  of  osmotic  activity  which 
has  thus  far  been  discovered  is  a  persistent  soaking  of  the  membranes 
in  water.  They  often  recover  under  this  treatment. 


190 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


^   "3 

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GLUCOSE. 


191 


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COM»O 


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cot^-^ 
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Os>—  ICOO3 


- 

s? 


192 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


pres 
Cal 


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196  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 

TABLE  67. — Osmotic  pressure  of  glucose  at  30°,  40°,  and  60°. 


Concentration. 

Osmotic  pressures. 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1.0 

At  30°. 

T2.473 

4.942 

7.417 

9.875 

12.361 

14.831 

17.326 

19.769 

22.259 

24.727 

Observed  pressures. 

]2.477 

4.957 

7.416 

9.885 

12.355 

14.842 

17.324 

19.771 

22.300 

24.693 

1  

9.894 

12.339 

17.331 

22.286 

24.767 

Mean  pressures  .... 

2.475 

4.950 

7.417 

9.885 

12.352 

14  !  837 

17.327 

19.770 

22.282 

24.727 

Ratio  of  osmotic  to 

gas  pressure  

1.001 

1.001 

1.000 

1.000 

1.000 

1.000 

1.002 

0.999 

1.002 

1.000 

At  40°. 

T2.541 

5.109 

7.655 

10.184 

12.749 

15.317 

17.886 

20.436 

23.049 

25.570 

Observed  pressures. 

42.565 

5.115 

7.663 

10.225 

12.763 

15.333 

17.856 

20.385 

22.969 

25.508 

[ 

17.869 

22  .  995 

Mean  pressures  .... 

2.553 

5.112 

7.664 

10.205 

12.756 

15.325 

17.870 

20.411 

23.000 

25.533 

Ratio  of  osmotic  to 

gas  pressure  

1.000 

1.001 

1.001 

0.999 

1.000 

1.000 

1.000 

0.999 

1.000 

1.000 

At  50°. 
Observed  pressures. 

(2.633 

5.283 
5.267 

7.901 
7.917 

10.524 
10.538 

13.197 
13.198 

15.806 
15.810 

18.453 
18.439 

21.082 
21.007 

23.633 
23.665 

26.281 
26.366 

Mean  pressures.  .  .  . 

2.633 

5.275 

7.909 

10.531 

13.198 

15.808 

18.446 

21.045 

23.649 

26.342 

Ratio  of  osmotic  to 

gas  pressure  

0.999 

1.001 

1.001 

0.999 

1.002 

1.000 

1.000 

0.998 

0.997 

0.999 

The  foregoing  measurements  of  the  osmotic  pressure  of  glucose  indi- 
cate that,  between  30°  and  50°,  the  aqueous  solutions  of  this  substance 
obey  the  gas  laws,  since — if  we  employ  the  weight  or  solvent  normal 
system  in  making  the  solutions,  and  refer  the  theoretical  gas  pressure 
of  the  solute  to  the  volume  of  the  pure  solvent — the  ratio  of  observed 
osmotic  to  calculated  gas  pressure  is,  in  all  cases,  approximately  unity. 
Stated  in  another  way,  the  equation  of  van't  Hoff  for  very  dilute 
solutions,  PV  =  KT,  applies  to  concentrated  solutions  of  glucose  between 
30°  and  50°,  provided  we  allow  the  V  to  signify  the  volume  of  the  pure 
solvent,  instead  of  the  volume  of  the  solution. 

The  osmotic  pressure  of  glucose,  and  also  of  cane  sugar,  will  be  meas- 
ured at  60°,  70°,  80°,  and,  if  possible,  at  still  higher  temperatures. 


CHAPTER  X. 
MANNITE. 

DETERMINATIONS  OF  OSMOTIC  PRESSURE. 

According  to  the  very  careful  determinations  of  Loomis,*  the  molec- 
ular depression  of  the  freezing  points  of  the  0.1  to  0.5  weight-normal 
solutions  of  mannite  is  normal,  i.  e.,  1.85°.  For  this  reason  the  deter- 
mination of  the  osmotic  pressure  of  the  substance  is  of  especial  interest. 

It  was  found,  as  shown  in  Chapter  V,  that  the  osmotic  pressure  of 
cane-sugar  solutions,  up  to  and  including  25°,  can  be  calculated  from 
the  observed  abnormal  depressions  of  the  freezing  points,  but  not  for 
higher  temperatures.  At  and  below  25°,  the  ratio  of  osmotic  to  the 
estimated  gas  pressure  of  the  solute  was  the  same  for  each  concentra- 
tion of  solution  as  the  ratio  of  the  observed  to  the  theoretical  depression 
of  the  freezing  point.  At  temperatures  above  25°  the  ratios  of  osmotic 
to  gas  pressure — previously  constant  but  abnormally  high — began  to 
decline,  and  the  osmotic  pressures  of  the  solutions  could,  of  course,  no 
longer  be  correctly  calculated  from  the  depressions  of  the  freezing 
points.  Stated  in  another  way,  the  osmotic  pressures  of  cane  sugar 
solutions  between  0°  and  25°  are  abnormal  to  the  same  degree  as  the 
depressions  of  the  freezing  points,  but  not  at  any  higher  temperatures. 
At  some  temperature  above  25°,  the  ratio  of  osmotic  to  gas  pressure 
became  unity  and  constant.  The  osmotic  pressures  of  the  solutions 
could  then,  of  course,  be  correctly  derived,  not  from  the  observed,  but 
from  the  theoretical,  depressions  of  the  freezing  points,  i.  e.,  from  a 
molecular  depression  of  about  1.85°. 

In  view  of  the  relations  between  freezing  points  and  osmotic  pres- 
sure, which  were  found  to  hold  in  the  case  of  cane  sugar,  it  was  to  be 
presumed  that  the  ratio  of  osmotic  to  gas  pressure  in  the  case  of 
mannite  solutions  would  be  found  to  be  unity  at  all  temperatures. 

Unfortunately  the  solubility  of  mannite  in  water  is  limited,  the  0.5 
weight-normal  being  the  most  concentrated  solution  whose  pressures 
can  be  measured  at  low  temperatures.  Otherwise,  it  is  an  excellent 
substance  with  which  to  answer  the  question  whether  those  compounds 
which  exhibit  normal  freezing-point  depressions  may  also  be  expected 
to  exhibit  normal  osmotic  pressures,  i.  e.,  pressures  which  conform  to 
the  gas  laws.  It  is  readily  obtained  in  sufficient  quantity  for  an 
extended  investigation,  and  in  sufficiently  pure  condition. 

*Zeitschrift  fur  physikalische  Chemie,  32,  599. 

197 


198      OSMOTIC  PRESSURE  OP  AQUEOUS  SOLUTIONS. 

The  determinations  of  the  osmotic  pressures  of  cane  sugar  and  glucose 
presented  in  Chapters  VIII  and  IX  were  designated  as  "final"  in  order 
to  express  the  confidence  of  the  author  in  the  general  correctness  of 
the  results.  The  measurements  contained  hi  the  present  chapter  are 
not  so  designated,  because  one  essential  test  of  their  reliability  has 
not  been  applied  to  them:  It  was  not  proved  that  the  solutions  of 
mannite  maintained  perfectly  their  concentration  while  in  the  cells. 
It  was  easy  to  do  this  in  the  case  of  cane  sugar  and  glucose,  because 
slight  changes  hi  the  concentration  of  solutions  could  be  detected  and 
measured  by  the  polariscope;  but  in  that  of  mannite,  there  was  at  the 
time  no  convenient  analytical  method  available.  The ' '  interferometer ' ' 
made  by  Zeiss  has  since  been  introduced  for  use  with  optically  inactive 
substances,  and  will  be  employed  when  the  "final"  determinations  of 
the  osmotic  pressure  of  mannite  are  undertaken.  The  author's  reasons 
for  insisting  on  a  perfect  maintenance  of  concentration  while  the  solu- 
tions are  hi  the  cells,  as  an  indispensable  part  of  the  evidence  of  credi- 
bility, have  already  been  given,  and  they  still  seem  to  him  perfectly 
valid,  and  worthy  of  the  strongest  emphasis.  Nevertheless,  he  does 
not  wish  to  be  understood  as  intimating  that  any  great  amount  of 
suspicion  attaches  to  the  present  determinations  of  the  osmotic  pressure 
of  mannite,  because  of  the  absence  of  this  proof.  On  the  contrary,  he 
believes  the  results  to  be  quite  trustworthy. 

A  large  proportion  of  the  cells  which  were  used  with  mannite  had 
previously  been  employed  in  measuring  the  osmotic  pressure  of  lithium 
chloride.  The  effect  of  exposure  to  an  electrolyte  is  to  render  the 
membranes  sluggish.  Evidence  of  that  result  is  probably  to  be  seen 

(1)  in  the  long  time  consumed  by  the  cells  in  coming  to  equilibrium,  and 

(2)  in  the  considerable  thermometer  effects,  i.  e.,  in  the  rather  large  fluc- 
tuations in  pressure  from  day  to  day.     As  usual  when  membranes  of 
diminished  activity  are  employed,  the  cells  were  generally  allowed  to 
make  long  records  for  the  purpose  of  minimizing  thermometer  effects. 
It  is,  however,  not  certain,  in  this  instance,  that  the  remarkable  tardi- 
ness of  the  cells  in  coming  to  equilibrium  was  due  wholly  to  the  effect 
of  the  electrolyte  upon  the  membranes;  for  it  was  observed  that  certain 
cells,  whose  membranes  had  not  been  in  contact  with  an  electrolyte, 
were  likewise  very  slow  in  establishing  their  final  pressures.     It  has 
not  yet  been  determined  whether  this  was  due  to  the  hard  burning  of 
the  cells — in  effect  to  the  limited  area  of  the  membranes — or  to  some 
peculiarity  of  the  mannite  which  distinguishes  it  from  cane  sugar  and 
glucose,  or  other  non-electrolytes.     Hitherto,  we  have  had  no  reason 
to  suspect,  in  the  case  of  non-electrolytes,  that  the  activity  of  the  mem- 
brane varies  with  the  solute.    The  question  is  an  important  one,  and 
it  will  be  carefully  investigated. 


MANNITE. 


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201 


Observed  mean  daily  total  pressures. 

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202 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


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MANNITE. 


203 


TABLE  70.  —  Determinations  of  osmotic  pressure  at  30°. 
W.  N.  S.  =  Weight  normal  solution.  C.  G.  P.  =  Calculated  gas  pressure. 

Observed  mean  daily  total  pressures. 

II 

rf(  »O      •      •  O 

oo  os    •    •  T? 
o  o    •    •  eo 

Observed  mean  daily  total  pressures. 

00    03 
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II 

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CN  -C 

1^  0 

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rH   CN    (N 
rH   Ttl   CO 
TJH    Tt  t> 

CO 

eo 

0  0      •      •      • 

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co  co 

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co 

01 

OS  rH        •-. 
If)  CO       •       •       • 

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11 

IN  in 

rH    IO 

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X 

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d      d        d        d 

d      d      d        d        d 

204 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


a 

f 


Observed  mean  daily  total  pressures. 

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O           rH           rH                O                O 

The  pressures  which  are  inclosed  in  parentheses  are  regarded  with  some  suspicion.  That  on  the  thirty-fourth  day  has  the  appearance  of  a  thermometer 
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10 

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205 


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at  40°  C. 

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206 


OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 


TABLE  71.  —  Determinations  of  osmotic  pressure  at  40°  —  Continued. 

Observed  mean  daily  total  pressures. 

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MANNITE. 


207 


Measurements  of  the  osmotic  pressure  of  mannite  have  been  made 
at  10°,  20°,  30°,  and  40°.  At  the  three  lower  temperatures,  the  0.1,  0.2, 
0.3,  0.4,  and  0.5  weight-normal  solutions  were  investigated.  At  40°, 
the  increased  solubility  of  mannite  in  water  made  it  practicable  also 
to  measure  the  osmotic  pressure  of  the  0.6  normal  solution. 


TABLE  72. — Osmotic  pressures  of  mannite  at  10° 


30°,  40° 


Osmotic  pressures. 

Concentration. 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

At  10°. 
Observed  pressures  

I  2.322 
\  2.310 
2.314 
1.002 

\  2.391 
j   2.398 

| 
4.609 

6.950 
6.930 
6.940 
1.002 

7.169 
7.192 

9.201 
9.216 
9.209 
0.997 

9.613 
9.572 
9.524 

11.613 

Mean  pressures  

4.609 
0.998 

4.778 
4.783 

11.613 
1.006 

11.929 
11.991 

Ratios  of  osmotic  to  gas  pressures 

At  20°. 
Observed  pressures  

Mean  pressures    

2.395 
1.002 

f  2.458 
j   2.479 

4.781 
1.000 

4.940 
4.946 

7.181 
1.001 

7.426 
7.434 

9.570 
1.001 

9.803 
9.949 
9.891 

11.960 
1.001 

12.326 
12.363 

Ratios  of  osmotic  to  gas  pressures 

At  30°. 
Observed  pressures  

Mean  pressures  

2.467 
0.999 

f  2.557 

4.943 
1.000 

5.103 
5  111 

7.430 
1.002 

7.664 

9.881 
0.999 

10.230 
10.201 
10.216 
1.000 

12.345 
0.998 

12.792 
12.816 
12.804 
1.003 

Ratios  of  osmotic  to  gas  pressures 

At  40°. 
Observed  pressures  

15.319 

Mean  pressures  

2.557 
0.998 

5.107 
1.000 

7.664 
1.001 

15.319 
1.000 

Ratios  of  osmotic  to  gas  pressures 

The  results  are  summed  up  hi  Table  73,  hi  which  are  given  the  mean 
osmotic  pressures  of  mannite  solutions,  between  10°  and  40°,  and  the 
ratios  of  these  pressures  to  the  calculated  gas  pressures  of  the  solute. 
It  will  be  seen  that  all  the  ratios  approach  unity,  showing  that,  within 
the  limits  thus  far  investigated,  the  aqueous  solutions  of  mannite  obey 
the  laws  of  Gay-Lussac  and  Boyle. 

TABLE  73. 


10C 

20C 

i 

30< 

> 

40< 

> 

tration. 

Osmotic 

Ratio. 

Osmotic 

Ratio. 

Osmotic 

Ratio. 

Osmotic 

Ratio. 

pressures. 

pressures. 

pressures. 

pressures. 

0.1 

2.314 

1.002 

2.395 

1.002 

2.467 

0.999 

2.557 

0.998 

0.2 

4.609 

0.998 

4.781 

1.000 

4.943 

1.000 

5.107 

1.000 

0.3 

6.940 

1.002 

7.181 

1.001 

7.430 

1.002 

7.664 

1.001 

0.4 

9.209 

0.997 

9.570 

1.001 

9.881 

0.999 

10.216 

1.000 

0.5 

11.613 

1.006 

11.960 

1.001 

12.345 

0.998 

12.804 

1.003 

0  6 

15.315 

1.000 

CHAPTER  XL 
ELECTROLYTES. 

It  has  already  been  intimated  that  much  of  the  conduct  of  osmotic 
membranes,  which  might  otherwise  appear  mysterious  and  capricious, 
becomes  explicable,  if  one  regards  the  membranes  as  having  a  purely 
colloidal  structure.  This  provisional  view  of  their  character  has  been  of 
great  utility  as  a  working  hypothesis  throughout  the  present  inves- 
tigation, inasmuch  as  it  was  only  by  proceeding  in  accordance  with 
its  suggestions  that  we  have  been  able  to  obtain,  and  to  maintain  in 
efficient  condition,  membranes  which  were  truly  semi-permeable  and 
therefore  adequate  for  the  measurement  of  osmotic  pressure.  Much  that 
is  to  be  said  in  this  connection  has  been  stated,  and  in  some  instances 
strongly  emphasized,  in  previous  chapters,  particularly  in  Chapter  IV. 
But  the  question  of  the  structure  of  the  membrane  is  of  such  funda- 
mental importance  to  the  measurement  of  osmotic  pressure  that  the 
author  makes  no  apology  for  recalling  here  those  peculiarities  of  its 
behavior  which  suggest  that  its  structure  is  colloidal.  They  are : 

1.  The  destructive  effect  of  an  accumulation  of  alkaline  hydroxides  in 
the  cell  during  the  deposition  of  the  membrane  by  electrolysis. — This  is  not, 
in  itself,  convincing  evidence  of  the  colloidal  nature  of  the  membrane, 
but  it  acquires  some  weight  in  that  direction  when  it  is  observed  that 
the  deterioration  of  the  membrane,  under  the  circumstances,  can  not 
be  fully  accounted  for  by  the  solvent  action  of  the  alkali  upon  it,  but 
is  probably  due,  in  a  great  measure,  to  an  accumulation  of  the  cations 
in  the  membrane  material.    As  bearing  upon  this  phase  of  the  subject, 
we  will  mention  the  beneficial  results  which  are  obtained  (1)  by  greatly 
diluting  the  solution  of  potassium  ferrocyanide;  (2)  by  substituting  for 
the  potassium  salt,  lithium  ferrocyanide  whose  cation  is  much  less  harm- 
ful to  colloidal  structure;  and  (3)  by  employing  ferrocyanic  acid,  rather 
than  any  of  its  salts. 

2.  The  impossibility  of  forcing  the  resistance  of  any  membrane  above  a 
given  value  by  continued  electrolysis. — That  progress  is  stopped  in  this 
case  by  an  accumulation  of  potassium  in  the  membrane  is  made 
extremely  probable  by  the  fact  that,  after  soaking  the  membrane  in 
water  free  from  electrolytes  and  then  resuming  the  electrolysis,  a  much 
higher  resistance  is  obtained. 

3.  The  decline  in  resistance,  and  the  ultimate  ruin  of  the  membrane, 
which  result  from  a  too  long  continued  electrolysis. — This  also  points  to 
an  accumulation  of  potassium  in  the  membrane,  which  diminishes  and 
finally  destroys  its  semipermeable  character. 

4.  The  remedial  effect  of  soaking  in  pure  water  membranes  which,  from 
any  cause  whatsoever,  have  partially  lost  their  semi-permeable  character. — 
The  improvement  of  the  membranes  under  such  treatment  has  not  yet 

209 


210      OSMOTIC  PRESSURE  OP  AQUEOUS  SOLUTIONS. 

been  observed  to  reach  a  maximum.  It  has  been  stated  in  general 
terms  that  "the  longer  the  soaking  is  continued,  the  greater  is  found  to 
be  the  improvement  of  the  membranes";  also,  that  those  membranes 
which  have  remained  submerged  in  pure  water  through  the  three 
summer  months  are  usually  in  excellent  condition  for  the  resumption 
of  work  in  the  fall.  But  the  most  notable  demonstration  of  the  value 
of  water  as  a  restorative  was  observed  in  connection  with  certain  cells 
which,  after  having  been  used  for  some  tune  at  high  temperatures, 
were  allowed  to  cool  quickly  down  to  ordinary  temperatures.  The 
subsequent  history  of  these  cells  has  been  partly  told  in  earlier  chapters, 
but  not  all  of  it.  More  than  three  months  were  spent  in  trying  to 
restore  them  to  usable  state,  i.  e.,  to  reproduce  the  semi-permeable 
condition  of  the  membranes,  but  without  success.  They  have  since 
remained  in  water  continuously  up  to  the  present  time.  Occasionally, 
they  have  been  tested  as  to  the  state  of  the  membranes  by  setting  them 
up  with  solutions  of  cane  sugar  or  glucose.  At  the  end  of  twelve 
months,  one  of  the  cells  began,  to  our  surprise,  to  develop  and  to  main- 
tain the  full  osmotic  pressures  of  the  solutions.  During  the  following 
five  months,  two  others  were  found  to  be  in  suitable  condition  for  the 
measurement  of  pressure;  and  during  the  eighteenth  month,  several 
more  weije  brought  into  use.  The  most  obvious  explanation  of  the 
effect  of  pure  water  on  the  semi-permeable  state  of  the  membranes  is 
that  it  preserves  and  improves  their  colloidal  condition. 

5.  The  auto-degeneration  of  the  membranes. — By  ' '  auto-degeneration  " 
is  meant  the  loss  of  semi-permeability  which  is  observed  in  such  mem- 
branes as  the  ferrocyanide  of  zinc  and  the  ferrocyanide  of  manganese. 
These  are  moderately  active  in  the  beginning,  but  soon  become  less  so, 
and  within  a  short  time  they  lose  every  vestige  of  the  semi-permeable 
character.  To  the  term  "auto-degeneration,"  there  should,  perhaps, 
be  added  that  of  induced-degeneration  to  cover  a  phenomenon  which  is 
also  observed  in  the  case  of  zinc  ferrocyanide,  the  fact,  namely,  that  when 
the  membrane  has  once  lost  its  semipermeability,  all  later  deposits  of 
membrane  material  immediately  lose  their  osmotic  activity.  In  such 
cases  there  is  a  change  in  the  condition  of  the  material,  which  appears  to 
consist  hi  a  passage  from  the  colloidal  to  a  granular  or  crystalline  state. 
In  the  presence  of  water,  and  in  the  absence  of  electrolytes,  membranes 
consisting  of  the  ferrocyanide  of  copper,  nickel,  or  cobalt  do  not  appear 
to  be  subject  to  what  is  here  styled  "auto-degeneration." 

Persuaded  as  we  were  (by  the  large  amount  of  seemingly  pertinent 
evidence,  which  had  been  gathered  while  we  were  measuring  the  pressures 
of  non-electrolytes)  that  the  semipermeability  of  the  membranes  de- 
pends on  their  colloidal  condition,  and  that  the  possibility  of  measuring 
osmotic  pressure  depends  upon  the  maintenance  of  that  state — the 
attempt  to  measure  the  pressures  of  electrolytes  was  begun  with  great 
misgivings.  It  was,  nevertheless,  determined  to  test  the  copper  ferro- 
cyanide membrane  with  various  salts;  and  in  case  of  failure  to  institute 


ELECTROLYTES.  211 

a  search  for  other  membranes  less  susceptible  to  electrolytes.  It  is  pro- 
posed to  give  in  the  present  chapter  a  brief  account  of  our  experience 
with  electrolytes  during  the  early,  or  preliminary,  stages  of  that  in- 
vestigation. 

EXPERIMENT  1. 

The  first  trial  was  with  a  0.5  weight-normal  solution  of  potassium 
chloride.  The  cell  selected  was  an  unusually  mature  one.  It  had  been 
in  use  about  four  years,  and  had  never  failed,  when  properly  treated, 
to  give  a  reliable  measurement  of  the  osmotic  pressure  of  a  non-elec- 
trolyte. Through  long  use  and  frequent  reinforcement,  the  membrane 
had  acquired  a  very  high  resistance,  and  the  cell  required  a  long  time 
for  the  establishment  of  equilibrium  pressures.  Nevertheless,  because 
of  its  proved  reliability,  it  was  still  highly  prized  for  the  measurement 
of  the  pressures  of  concentrated  solutions,  in  which  large  thermometer 
and  barometer  effects  are  of  less  relative  importance  than  in  dilute 
solutions.  The  resistance  of  the  cell  at  the  time  of  setting  it  up  with 
the  solution  of  potassium  chloride  was  1,170,000  ohms,  and  the  temper- 
ature of  the  bath  was  30°.  The  initial  pressure  was  adjusted  to  about 
22.5  atmospheres.  There  followed  some  fluctuations  of  bath  tempera- 
ture and  the  mercury  meniscus  did  not  come  to  rest  until  the  fourteenth 
day.  The  indicated  osmotic  pressure  of  the  solution  at  that  time  was 
20.644  atmospheres.  Twenty  days  later,  it  was  20.679  atmospheres. 
The  intermediate  variations  in  pressure  were  small,  and  could  be  reason- 
ably ascribed  to  thermometer  and  barometer  effects.  On  the  thirty- 
fourth  day — while  still  at  full  equilibrium  pressure,  and  exhibiting  no 
signs  of  weakness — the  cell  was  opened.  The  water  in  which  the  cell 
had  stood  during  the  experiment  was  examined  for  chlorine.  The 
amount  found  was  equivalent  to  1.7  milligrams  per  100  cubic  centi- 
meters. But,  since  the  cell  is  always  somewhat  soiled  by  the  solution 
at  the  time  of  filling  it,  the  presence  of  this  small  quantity  of  chlorine 
was  not  believed  to  signify  leakage  on  the  part  of  the  membrane.  On 
the  whole,  this  determination  of  the  osmotic  pressure  of  the  0.5  weight- 
normal  solution  of  potassium  chloride  was,  and  still  is,  regarded  as 
probably  very  nearly  correct.  The  mean  of  the  osmotic  pressures  of 
the  first  and  last  days  of  equilibrium  is  20.662,  while  the  mean  of  all 
the  20  days  of  equilibrium  is  20.610.  The  theoretical  pressure,  calcu- 
lated— as  best  it  may  be — from  Kohlrausch's  values  for  the  dissociation 
of  potassium  chloride,  and  presuming  no  hydration  of  the  solute  to 
exist  which  modifies  the  osmotic  pressure,  is  22.110  atmospheres. 

It  was  now  to  be  determined,  by  means  of  a  series  of  repetitions  of  the 
experiment,  whether  the  membrane  had  suffered,  or  does  suffer,  any 
deterioration  in  consequence  of  contact  with  the  electrolyte.  Accord- 
ingly the  cell  was  soaked  in  water  for  six  days,  and  then  set  up  again 
with  another  0.5  weight-normal  solution  at  the  same  temperature. 
The  resistance  of  the  membrane  on  the  second  occasion  was  400,000, 
whereas  it  had  been  1,170,000  ohms  on  the  first  trial.  The  cell  re- 


212      OSMOTIC  PRESSURE  OF  AQUEOUS  SOLUTIONS. 

mained  in  the  bath  17  days,  and  the  highest  osmotic  pressure  exhibited 
by  the  solution  during  this  time  was  18.579  atmospheres.  In  general, 
the  pressure  showed  a  tendency  to  decline,  and  on  the  seventeenth  day 
it  had  fallen  to  17.407  atmospheres. 

In  the  third  trial  the  cell  was  soaked  in  water  8  days  and  then  set  up, 
as  on  previous  occasions,  with  a  0.5  weight-normal  solution  of  potassium 
chloride.  It  remained  in  the  bath  15  days.  The  highest  osmotic 
pressure  observed  was  12.515.  On  the  fifteenth  day,  the  pressure  had 
declined  to  10.386  atmospheres. 

The  membrane  had  no  doubt  suffered  severely  in  contact  with  the 
potassium  chloride.  The  nature  of  the  injury  is,  of  course,  indetermin- 
able; but  the  conduct  of  the  copper  ferrocyanide  membrane  in  the  pres- 
ence of  this  electrolyte  resembles  that  of  the  zinc  ferrocyanide  mem- 
brane in  contact  with  either  water  or  a  solution  of  a  non-electrolyte. 

EXPEBIMENT  2. 

Another  cell,  which  had  also  made  a  long  and  uniformly  good  record 
with  non-electrolytes,  was  set  up  at  30°  with  a  0.5  weight-normal  solu- 
tion of  potassium  chloride  and  allowed  to  remain  in  the  bath  36  days. 
The  osmotic  pressure  which  it  should  have  developed,  according  to 
the  record  of  the  cell  used  in  experiment  1,  was  about  20.6  atmospheres; 
the  highest  observed  pressure  was  18.567  atmospheres.  On  the  thirty- 
sixth  day,  the  osmotic  pressure  had  fallen  to  17.628  atmospheres. 

The  same  cell  was  soaked  in  water  for  20  days  and  set  up  again  under  the 
same  conditions  as  before.  It  remained  in  the  bath  24  days.  The  highest 
osmotic  pressure  exhibited  by  the  solution  was  16.695  atmospheres.  The 
pressure  on  the  twenty-fourth  day  was  16.299  atmospheres. 

The  pressure  on  the  first  trial  was  greater  in  experiment  1  than  hi 
experiment  2,  showing  that  the  membrane  in  the  former  case  was 
originally  the  better  of  the  two;  but  the  deterioration  at  the  end  of  the 
second  trial  was  relatively  less  in  experiment  2  than  in  experiment  1. 
This  was  probably  due  to  the  much  longer  soaking  in  water  between 
trials  which  was  given  to  the  membrane  in  the  second  cell. 

Eight  other  experiments,  in  most  respects  similar  to  experiments  1 
and  2,  were  carried  out  with  cells  whose  excellence  for  the  measurement 
of  the  osmotic  pressure  of  non-electrolytes  had  been  fully  demonstrated, 
but  as  no  further  light  was  obtained  through  them,  as  to  the  action  of 
electrolytes  on  the  membrane,  they  are  omitted.  The  results  were  all 
confirmatory  of  the  observations  made  in  experiments  1  and  2,  and  to 
the  effect  that  the  copper  ferrocyanide  membrane  suffers  severely  in 
the  presence  of  potassium  chloride.  The  question,  whether  the  ten 
membranes  which  have  been  injured  by  this  electrolyte  can  be  restored 
to  usefulness  by  long-continued  soaking  in  water,  is  still  to  be  answered. 

Two  experiments  were  made  with  0.5  weight-normal  solutions  of 
barium  chloride,  but  the  results  were  as  unsatisfactory  as  had  been  those 
with  potassium  chloride. 


ELECTROLYTES.  213 

The  supposed  "protective"  action  of  such  colloids  as  albumen  and 
gelatin  was  also  tested  with  the  chlorides  of  potassium  and  barium,  but 
without  discoverable  advantage  to  the  membranes. 

One  experiment  was  carried  through  with  potassium  ferrocyanide, 
but  the  membrane  gave  the  same  unmistakable  evidence  of  deteriora- 
tion under  the  influence  of  this  electrolyte  that  it  had  exhibited  when 
tested  with  potassium  chloride. 

A  few  experiments  were  made  with  potassium  chloride  in  cells  having 
membranes  of  nickel  ferrocyanide.  Membranes  of  this  material  are 
probably  somewhat  superior  to  those  of  copper  ferrocyanide  for  the 
measurement  of  the  pressures  of  non-electrolytes.  They  were  found, 
however,  to  have  no  advantage  over  the  latter  for  use  with  electrolytes. 

Some  evidence  was  gathered  to  the  effect  that  it  will  be  possible  to 
measure  the  osmotic  pressure  of  quite  dilute  solutions  of  potassium 
salts,  even  with  the  copper  ferrocyanide  membrane.  This  is  of  interest 
in  connection  with  the  fact,  as  will  be  shown  later,  that  the  practicability 
of  measuring  the  osmotic  pressure  of  lithium  salts  is  altogether  a  ques- 
tion of  concentration. 

When  cells  of  demonstrated  excellence  were  set  up  with  half  normal 
solutions  of  potassium  chloride,  there  was  always  obtained  on  the  first 
trial  a  high  pressure.  On  one  occasion,  it  was  probably  the  maximum 
osmotic  pressure  of  the  solution.  In  all  succeeding  trials,  however, 
smaller  and  smaller  pressures  were  obtained,  until,  in  some  instances, 
the  pressure  observed  on  the  third  trial  had  fallen  to  about  one-half  of 
its  first  value.  Such  conduct  on  the  part  of  the  cells  can  only  be 
explained  by  supposing  that  the  membranes  had  degenerated  to  the 
point  of  becoming  quite  permeable  to  the  solute,  and  one  would  expect, 
perhaps,  to  find  considerable  chlorine  in  the  water  in  which  the  cells 
had  stood.  As  a  matter  of  fact,  however,  the  amount  of  it  which  made 
its  way  into  the  water  surrounding  the  cells  was  very  small,  and  in  no 
instance  sufficient  to  account  for  more  than  a  minute  fraction  of  the 
deficit  in  pressure.  This  observation  is  cited  here  in  order  to  empha- 
size again  the  fact — already  more  than  once  stated — that  it  is  useless 
to  attempt  to  measure  osmotic  pressure  in  leaky  cells;  because  the 
escaped  solute  always  concentrates  heavily  in  the  pores  of  the  cell  wall, 
giving  a  solution  of  wholly  unknown  concentration  in  contact  with  the 
exterior  surface  of  the  membrane.  Such  concentration  may  be  due  in 
part  to  lack  of  time  for  diffusion,  but  it  is  probably  due  in  much  greater 
measure  to  adsorption. 

In  general,  high  resistance  is  regarded  as  a  good  sign  in  a  membrane; 
but  it  is  certainly  no  proof  of  its  ability  to  measure  osmotic  pressure, 
if  the  membrane  has  once  suffered  injury  from  contact  with  electrolytes. 
In  some  later  experiments  with  potassium  chloride,  where  the  cells 
were  unable  to  develop  even  half  the  normal  pressures  of  the  solutions, 
the  membranes  had  still  a  resistance  of  more  than  a  half  million  ohms. 


214  OSMOTIC   PRESSURE    OF   AQUEOUS   SOLUTIONS. 


DETERMINATIONS  OF  THE  OSMOTIC  PRESSURE  OF  LITHIUM  CHLORIDE  AT  30°.* 

It  has  been  observed  that  the  lithium  salts  appear  to  be  much  less 
harmful  to  the  membranes  than  those  of  potassium.  Abundant  evi- 
dence of  this  will  appear  in  the  following  record  of  the  determina- 
tions of  the  osmotic  pressure  of  lithium  chloride  solutions,  ranging  in 
concentration  from  0. 1  to  0.6  weight-normal.  The  superior  resistance  of 
the  membranes  to  salts  of  lithium  is  possibly  due  to  the  large  atmos- 
phere of  water  with  which  the  cation  is  supposed  to  be  surrounded. 

The  determinations  of  the  osmotic  pressure  of  lithium  chloride  here 
recorded  are  not  regarded  as  "final,"  because  it  was  not  demonstrated 
that  the  solutions  maintained  perfectly  their  concentration  while  in  the 
cells.  They  will,  therefore,  be  repeated  at  a  later  date  when  an  "inter- 
ferometer" is  available  for  the  purpose  of  detecting  and  measuring 
slight  differences  in  concentration.  They  are  believed,  however,  to  be 
very  nearly  correct.  All  the  water  in  which  the  cells  had  stood  during 
the  experiments  was  examined  for  the  presence  of  chlorine.  In  every 
case  a  slight  milky  appearance  was  produced  by  silver  nitrate,  but  the 
quantity  of  chlorine  thus  precipitated  did  not  in  any  instance  exceed 
2  milligrams  per  100  cubic  centimeters  of  the  solution,  and  that  amount 
could  be  accounted  for  as  due  to  a  slight  unavoidable  soiling  of  the  cell 
while  filling  it  with  the  solution.  The  reasonableness  of  this  explana- 
tion was  confirmed  by  the  fact  that  the  quantities  of  chlorine  found 
bore  no  definite  relation  to  the  duration  of  the  experiments.  After 
leakage,  the  most  frequent  cause  of  a  change  in  the  concentration  of 
the  cell  contents  is,  as  previously  stated,  an  improper  adjustment  of 
the  initial  pressure.  But  such  adjustments  give  very  little  trouble, 
except  at  high  temperatures,  and  they  are  not  believed  to  have  affected 
the  concentration  of  the  solutions  of  lithium  chloride  at  30°.  A  third, 
but  at  present  infrequent,  cause  of  dilution  or  concentration  of  the  cell 
contents  has  not  been  previously  mentioned.  It  depends  on  the  use 
of  rubber  in  closing  the  cells.  The  employment  of  this  material  is, 
unfortunately,  essential  to  the  proper  adjustment  of  initial  pressure, 
but  its  use  carries  with  it  the  danger  that,  through  its  movements  under 
pressure,  the  capacity  of  the  cell,  and  therefore  the  concentration  of  the 
solution,  may  be  altered.  The  movement  of  the  rubber  may  be  inward, 
and  result  in  concentration;  or  outward,  and  result  in  dilution.  Every 
effort  is  made  to  confine  the  rubber  in  such  a  manner  as  to  reduce  its 
possible  movements  to  the  limits  essential  to  the  proper  adjustment  of 
initial  pressure,  but  the  means  of  effecting  this  has  always  been  one  of 
the  more  perplexing  mechanical  problems  of  the  present  investigation. 
If  the  solutions  of  lithium  chloride  failed  to  maintain  perfectly  their 
concentration,  it  was  probably  due  to  some  imperfect  confinement  of 
the  rubber  which  was  employed  in  closing  the  cells. 

*Measurements  by  H.  N.  Morse,  J.  C.  W.  Frazer,  and  E.  L.  Frederick. 


ELECTROLYTES. 


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216 


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ELECTROLYTES. 


217 


The  pressures  which  were  obtained  are  very  large  as  compared  with 
the  calculated  gas  pressure  of  molecular  lithium  chloride,  the  ratios 
being  (see  table  75)  1.746, 1.816, 1.857,  1.899,  1.955,  and  1.992,  respec- 
tively, for  the  0.1,  0.2,  0.3,  0.4,  0.5,  and  0.6  weight-normal  solutions. 
Such  excessive  pressures  were,  of  course,  to  be  expected  from  the 
known  considerable  electrolytic  dissociation  of  the  salt;  but  the  ratios 
cited  above  do  not  diminish  with  increasing  concentration,  as  would 
be  expected,  if  the  differences  between  the  observed  osmotic  and  the 
calculated  gas  pressures  were  due  solely  to  electrolytic  dissociation. 
On  the  contrary,  the  ratios  in  question  increase  in  value  with  increasing 
concentration.  A  similar  increase  in  ratio  of  osmotic  to  calculated  gas 
pressure  was  observed  in  the  case  of  cane-sugar  solutions,  and  it  was 
tentatively  ascribed  to  a  hydration  of  the  solute.  It  was  presumed,  in 
other  words,  that  any  withdrawal  of  solvent  molecules  for  the  purpose 

TABLE  75. — Osmotic  pressure  of  lithium  chloride  at  SO0. 


Concentration. 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

Observed  pressures  

/4.325 
\4.311 
4.317 
2.472 
1.746 

8.946 
9.005 
8.976 
4.943 
1.816 

13.809 
13.626 
13.768 
7.415 
1.857 

18.755 
18.789 
18.772 
9.886 
1.899 

24.162 

29.535 

Mean  pressures  

24.162 
12.358 
1.955 

29.535 
14.830 
1.992 

Calculated  gas  pressures*  

Ratio  of  osmotic  to  gas  pressure  

*  For  undissociated  salt. 

of  hydrating  those  of  the  solute  would  have  the  effect  of  concentrating  a 
solution,  and  that  the  result  of  such  concentration  would  be  an  apparently 
abnormally  high  osmotic  pressure.  Concentration  through  hydration  of 
the  solute  should  manifest  itself  in  the  form  of  an  increasing  ratio  of 
osmotic  to  gas  pressure,  such  as  was  observed  in  the  case  of  lithium 
chloride  at  30°  and  in  that  of  cane  sugar  at  the  lower  temperatures. 

The  electrolytic  dissociation  of  the  solutions  of  lithium  chloride, 
whose  osmotic  pressures  were  measured,  have  not  yet  been  determined, 
and  the  data  available  are  insufficient  for  a  safe  estimation  of  the  same. 
It  is,  therefore,  impossible  to  say  at  present  what  proportion  of  the 
observed  difference  between  osmotic  and  gas  pressure  is  to  be  ascribed, 
on  the  one  hand,  to  dissociation;  and,  on  the  other,  to  a  conjectured 
concentration  of  the  solution  through  hydration  of  the  solute.  If  the 
whole  difference  is  ascribed  to  dissociation,  the  percentages  of  ionized 
salt  which  must  be  presumed  to  exist  in  the  0.1,  0.2,  0.3,  0.4,  0.5,  and 
0.6  weight-normal  solutions  of  lithium  chloride  are  74.6,  81.6,  85.7, 
89.9,  95.5,  and  99.2  respectively.  Such  relations  of  dissociation  to 
concentration  are,  of  course,  quite  impossible. 

The  effect  which  was  produced  upon  the  membranes  by  the  lithium 
chloride  was  very  pronounced.  They  became  at  once  exceedingly  slug- 


218  OSMOTIC   PRESSURE    OF   AQUEOUS    SOLUTIONS. 

gish.  Hitherto,  diminished  activity  on  the  part  of  membranes  has 
usually  been  the  result  of  age  and  frequent  use.  But  the  membranes 
which  were  employed  for  the  measurement  of  the  osmotic  pressure  of 
the  lithium  salt  were  new  ones,  and  they  had  not  been  used  for  any 
other  purpose.  With  either  cane-sugar  or  glucose  solutions,  they  should 
have  given  equilibrium  pressures  within  one  or  two  days.  With  solu- 
tions of  lithium  chloride,  the  shortest  time  required  for  that  purpose 
was  9  days,  while  the  average  time  consumed  in  developing  the  final 
pressures  was  17  days.  The  membranes  were  not  wholly  ruined  by 
their  contact  with  the  electrolyte,  as  others  had  been  by  potassium 
chloride;  for  they  were  afterwards  successfully  employed  for  the  meas- 
urement of  the  osmotic  pressure  of  mannite  solutions.  But  the  state 
of  inertness  which  they  had  acquired  in  the  presence  of  the  lithium  salt 
persisted  without  diminution  throughout  their  later  history.  Event- 
ually, the  cells  were  withdrawn  from  use,  because  of  their  slowness,  and 
consigned  to  a  solution  of  thymol,  in  order  to  ascertain  whether  the 
membranes  might  not  recover  their  normal  activity  under  the  influ- 
ence of  water.  This  is  the  course  which  is  now  taken  with  all  slow 
cells  whenever  their  long-continued  monopolization  of  bath  space  and 
manometers  becomes  intolerable.  Many  membranes  do  recover  a  fair 
degree  of  activity  under  such  treatment,  though  the  time  required  for 
restoration  is  usually  very  long — sometimes  more  than  two  years. 

Particular  attention  is  called  to  experiment  2  with  the  0.4  weight- 
normal  solution.  This  was  an  endurance  test  of  the  membrane  of  an 
unusually  thorough  character.  The  cell  (F6),  at  the  time  of  setting 
it  up,  had  a  resistance  of  1,100,000  ohms,  and  it  remained  in  the  bath 
145  days.  Starting  with  an  initial  pressure  of  15  atmospheres,  it 
reached  an  approximate  equilibrium  in  10  days.  The  osmotic  pressure 
which  the  cell  sustained  during  the  following  125  days  is  given  in  5 
columns,  each  of  25  daily  records.  The  mean  osmotic  pressure  for 
the  first  period  was  18.827;  for  the  second,  18.894;  for  the  third,  18.799; 
for  the  fourth,  18.636;  and  for  the  fifth,  18.405.  It  is  believed  that  a 
mean  of  the  records  for  the  first  100  days  fairly  represents  the  osmotic 
pressure  of  the  solution.  But  during  the  fifth  period,  i.  e.}  from  the 
101st  to  the  125th  day  of  the  record,  there  was  a  decline  in  pressure  from 
18.609  to  18.140  atmospheres,  which  can  only  signify  that  the  membrane 
had  at  last  begun  to  weaken.  The  cell  was  allowed  to  remain  10  days 
longer  in  the  bath,  but  it  gave  no  evidence  of  recovering  any  portion  of 
the  loss  sustained  during  the  fifth  period;  in  fact,  the  rate  of  decline  in 
pressure  increased  quite  perceptibly.  The  membrane  of  cell  F6  had 
evidently  suffered  severely  from  its  long  contact  with  lithium  chloride; 
for  it  was  found  unfit  for  further  measurements  of  pressure.  This  does 
not  mean,  of  course,  that  it  can  never  be  restored  to  usefulness. 

The  very  considerable  resistance  of  the  membrane  to  the  electrolyte, 
which  was  exhibited  in  the  case  of  the  endurance  experiment  with  the 


ELECTROLYTES. 


219 


0.4  weight-normal  solution  of  lithium  chloride,  encouraged  the  hope 
that  it  would  be  found  practicable  to  measure  the  osmotic  pressure  of 
much  more  concentrated  solutions  of  that  salt.  But  when  we  proceeded 
to  the  investigation  of  the  higher  concentrations,  it  was  found  that  the 
injury  to  the  membranes  by  the  electrolyte  increased  rapidly  with 
increasing  concentration  of  solution.  The  pressure  of  the  0.5  and  0.6 
weight-normal  solutions  were  successfully  measured,  but  only  by  the 
sacrifice  of  two  of  the  best  cells  in  our  possession.  It  was  not  possible 
to  duplicate  these  determinations  with  any  other  cells  which  were 
available  at  that  time.  The  effect  of  lithium  chloride  upon  the  copper 
ferrocyanide  membrane  appears  to  be  milder  than  that  of  potassium 
chloride,  but  not  different  in  kind. 

TABLE  76. 


I. 

II. 

III. 

IV. 

V. 

18.731 

18.979 

18.925 

18.655 

18.609 

18.671 

18.883 

18.912 

18.646 

18.582 

18.677 

18.880 

18.912 

18.521 

18.552 

18.720 

18.862 

18.939 

18.669 

18.509 

18.768 

18.880 

18.789 

18.672 

18.486 

18.769 

18.918 

18.820 

18.692 

18.470 

18.751 

18.937 

18.812 

18.708 

18.497 

18.793 

18.959 

18.708 

18.710 

18.515 

18.768 

18.935 

18.818 

18.717 

18.559 

18.827 

18.907 

18.792 

18.698 

18.563 

18.840 

18.882 

18.743 

18.681 

18.552 

18.843 

18.885 

18.933 

18.675 

18.521 

18.711 

18  888 

18.829 

18.676 

18.417 

18.889 

18.880 

18.785 

18.587 

18.401 

18.897 

18.881 

18.773 

18.621 

18.391 

18.863 

18.859 

18.750 

18.621 

18.380 

18.857 

18.907 

18.736 

18.609 

18.345 

18.898 

18.853 

18.735 

18.561 

18.240 

18.892 

18.863 

18.739 

18.568 

18.291 

18.910 

18.913 

18.739 

18.611 

18.295 

18.992 

18.869 

18.731 

18.610 

18.225 

18.898 

18.880 

18.759 

18.593 

18.247 

18.917 

18.869 

18.770 

18.585 

18.235 

18.890 

18.799 

18.778 

18.602 

18.113 

18.896 

18.993 

18.745 

18.609 

18.140 

18.827 

18.894 

18.799 

18.636 

18.405 

Mean  osmotic  pressure  for  100  days,  18.789. 

The  conclusions  to  be  drawn  from  the  experiences  thus  far  reported 
are :  (1)  that  it  is  practicable  to  measure  the  osmotic  pressure  of  lithium 
chloride  in  all  aqueous  solutions  not  more  concentrated  than  the  0.6 
weight-normal;  (2)  that  it  will  probably  be  found  possible  to  measure 
the  osmotic  pressure  of  potassium  chloride  in  aqueous  solutions  less 
concentrated  than  the  0.5  weight-normal. 

It  is  hoped  that  other  semi-permeable  membranes  may  be  found 
which  are  less  susceptible  to  the  deleterious  influence  of  electrolytes 
than  are  the  ferrocyanides  of  copper  and  nickel. 


CHAPTER  XII. 
CONCLUSION. 

The  work  reported  upon  in  the  preceding  chapters  is  only  a  fraction 
of  the  task  which  the  author  hopes  to  accomplish,  or  to  see  accomplished 
by  others.  The  investigation — already  15  years  old — was  undertaken, 
in  the  first  instance,  with  a  view  to  developing  a  practicable  and  fairly 
precise  method  for  the  direct  measurement  of  the  osmotic  pressure  of 
aqueous  solutions.  The  need  of  such  a  method  for  the  investigation 
of  solutions  seemed  to  the  author  very  great  and  very  urgent.  The 
freezing-  and  boiling-point  methods  were  of  great  value,  but  of  limited 
applicability,  in  that  they  could  give  no  certain  information  as  to  the 
conditions  within  a  solution,  except  at  two  widely  separated  and  rather 
exceptional  temperatures.  There  appeared  to  be  a  need  of  more  com- 
prehensive methods — of  methods  which  could  be  effectively  applied 
to  the  investigation  of  solutions  at  all  temperatures  between  the  freezing 
and  boiling  points.  Two  such  methods  naturally  suggested  themselves . 
One  of  these  was  a  method  for  the  direct  determination  of  the  osmotic 
pressure,  and  the  other  was  a  method  for  the  measurement  of  the 
depression  of  the  vapor  tension  of  solutions.  Neither  had  been  per- 
fected to  a  point  where  it  could  be  made  to  yield  convincing  results. 
The  method  selected  by  the  author  for  development  was  that  for  the 
measurement  of  osmotic  pressure.  Nearly  eight  years  were  devoted 
to  one  or  another  phase  of  this  part  of  the  enterprise.  The  difficulties 
which  were  encountered  during  the  evolution  of  the  method  were  great, 
and  often  they  were  baffling  and  for  long  periods  seemingly  insurmount- 
able. Fortunately  for  the  undertaking,  it  was  adopted  by  the  Carnegie 
Institution  of  Washington  as  soon  as  it  became  apparent  that  the 
problems  involved  would  require  many  years  and  large  means  for  their 
effective  solution.  It  was  also  fortunate  for  the  enterprise  that  the 
author  has  had  associated  with  him  during  the  greater  part  of  the  time 
two  such  able  and  tireless  coadjutors  as  Dr.  J.  C.  W.  Frazer  and  Dr.  W. 
W.  Holland,  whose  resourcefulness  has  contributed  much  to  whatever 
success  has  been  attained.  The  development  of  the  method,  which  is 
described  in  the  earlier  chapters  of  this  report,  is  now  regarded  as 
reasonably  complete — inasmuch  as,  in  the  hands  of  experienced  persons, 
it  can  be  made  to  yield  results  which  compare  favorably  with  those  of 
other  and  simpler  quantitative  operations. 

Having  perfected  the  method,  it  was  to  be  applied  to  the  measure- 
ment of  osmotic  pressure  in  accordance  with  a  systematic  plan.  It 
was  determined  to  measure  with  all  possible  care  the  pressures  of  four 
substances  over  a  wide  range  of  concentration  and  temperature.  The 

221 


222  CONCLUSION. 

compounds  selected  for  the  purpose  were  cane  sugar,  glucose,  levulose, 
and  mannite.  Some  of  the  reasons  for  this  choice  of  materials  are 
given  below : 

(1)  All  four  of  the  compounds  named  separate  from  solution  without 
water  of  crystallization,  which  simplifies  the  situation  by  making  it 
possible,  for  a  time,  to  evade  the  question  whether  such  water,  when 
a  substance  is  dissolved,  belongs  to  the  solute  or  to  the  solvent. 

(2)  The  list  includes  substances  which  are  both  normal  and  also,  in 
different  ways,  abnormal  in  respect  to  their  freezing-point  depressions. 
These  compounds  therefore  afford  an  excellent  opportunity  for  com- 
paring experimentally  determined  osmotic  pressures  with  a  variety  of 
freezing-point  depressions. 

(3)  Three  of  the  compounds  are  optically  active,  and  alterations  in 
the  concentration  of  their  solutions  can  be  readily  detected  and  meas- 
ured by  the  polariscope.     Mannite,  the  fourth  substance,  was  selected, 
notwithstanding  its  optical  inactivity,  because  the  depression  of  the 
freezing  points  of  its  solutions  are  all  normal;  and,  since  the  intro- 
duction of  the  interferometer,  the  lack  of  optical  activity  is  no  longer 
an  objection  to  it. 

It  was  proposed  to  measure  the  pressures  of  the  enumerated  sub- 
stances from  0°  to  the  highest  temperature  at  which  it  is  practicable 
to  work— possibly  to  100°.  It  was  thought  that,  by  extending  the 
investigation  over  a  wide  range  of  temperature,  much  light  might  be 
obtained  on  the  problem  of  hydration  and  its  relation  to  the  freezing- 
point  depressions  and  osmotic  pressures  of  solutions.  The  work  is  now 
in  its  second  stage — in  that  stage,  namely,  in  which  the  osmotic  pressure 
of  cane  sugar,  glucose,  levulose,  and  mannite  is  under  investigation. 
About  three  years  more  will  be  required  to  complete  the  proposed 
study  of  these  substances. 

Having  finished  the  investigation  of  the  anhydrous  compounds  men- 
tioned above,  it  is  proposed  to  study,  in  a  similar  manner,  several  of 
the  carbohydrates  which  separate  from  solution  with  water  of  crystal- 
lization. It  is  also  proposed  to  continue  the  investigation  of  the 
osmotic  pressure  of  electrolytes. 

Lists  of  those  osmotic  pressures  which  the  author  regards  as  estab- 
lished with  a  reasonable  degree  of  certainty  are  to  be  found : 

(1)  For  cane  sugar,  in  Tables  59,  60,  and  62,  pages  184  and  186. 

(2)  For  glucose,  in  Table  67,  page  196. 

(3)  For  mannite,  in  Tables  72  and  73,  page  207. 

The  conclusions  which  were  drawn  from  them  have  been  sufficiently 
discussed  from  time  to  time  in  the  course  of  this  report. 


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